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Designing with ESBs in
APEX II Devices
March 2002, ver. 1.0
Introduction
Application Note 179
In APEXTM II devices, enhanced embedded system blocks (ESBs) support
memory structures, such as single-port and dual-port RAM. Additionally,
in APEX II devices, ESBs support bidirectional dual-port RAM. In this
mode, each ESB has two ports that allow two different read or write
operations simultaneously. In APEX II devices, the ESB can be split into
two separate single-port 2K RAM blocks.
This application note explains the basic operation of the APEX II ESB, how
to implement different memory configurations in ESBs, some applications
using ESBs, and the initialization file in the Altera Quartus® II software.
This application note also includes a timing diagram for each memory
configuration type.
ESB Memory
Configurations
& Features
This application note discusses the following ESB memory configurations:
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■
single-port RAM
dual-port RAM
dual-port+ RAM (bidirectional dual-port RAM)
ROM
FIFO
Single-Port RAM Configuration
The single-port RAM configuration is shown in Figure 1 and has the
following features:
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Altera Corporation
A-AN 179-1.0
A-AN-179-1.0
Uses one port that supports either one read or one write.
Read during write always uses the same address for reading and
writing.
The ESB only uses the write clock. As a result, the output depends on
the ESB synchronous write cycle time, which is slower than the read
cycle time. Therefore, the read cycle runs at a slower frequency as
compared to the dual-port RAM configuration.
Data bus widths definable as 1, 2, 4, 8, or 16 bits for input and output
buses.
Registered inputs and outputs each have clock, clock enable, and
clear ports.
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Figure 1. Single-Port Memory Configuration in an ESB
■
Partitioning the ESB into two separate 2K-bit single-port RAM blocks
increases the total number of memory blocks per device. Each 2K
RAM block has its own read and write port, but the maximum
data-width is 8-bits. The 2K RAM configurations are 256 × 8, 512 × 4,
1024 × 2 and 2048 × 1. See Figure 2. The 2K RAM divisions must have
the same configuration.
Figure 2. Single-Port Partitioned RAM in an ESB
Dual-Port RAM Configuration
The dual-port RAM configuration is shown in Figure 3 and has the
following features:
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■
■
2
The ESB can simultaneously read and write data. The ESB supports
variable-width data ports for reading and writing to the RAM. For
example, the ESB can be written in 1-bit mode at port A while being
read in 16-bit mode from port B. Table 1 lists the supported variable
width configurations for an ESB in dual-port mode.
The read and write address ports have independent clock, clock
enable, and clear ports.
Uses both the read and write clock. When performing synchronous
read and writes, the read cycle time is faster than the write cycle time.
The output depends on the read cycle time, so the performance is
higher than for the single-port RAM configuration, which depends
on the write cycle time.
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Figure 3. Dual-Port RAM Memory Configuration in an ESB
Table 1. Variable Width Configuration for Dual-Port RAM
Read Port Width
Write Port Width
1 bit
2 bits, 4 bits, 8 bits, or 16 bits
2 bits, 4 bits, 8 bits, or 16 bits
1 bit
Bidirectional Dual-Port RAM Configuration
The bidirectional dual-port RAM configuration is shown in Figure 4 and
has the following features:
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■
■
■
Two independent ports, port A and port B
Can simultaneously perform two reads, two writes, or one read and
one write
Data bus width definable as 1, 2, 4, or 8 bits
Independent clock, clock enable, and clear set signals for port A and
port B
Figure 4. Bidirectional Dual-Port RAM Configuration in an ESB
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FIFO Buffer Configuration
The FIFO buffer configuration is shown in Figures 5 and 6 and has the
following features:
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■
Data transferred between design subsystems uses FIFO memory
buffers. For instance, FIFO buffers hold data that is driven from
multiple sources to a shared bus. When the bus is busy, the FIFO
buffer stores the data; when the bus is not busy, the FIFO buffer sends
the data to the bus.
An ESB can implement a single- or dual-clock FIFO to buffer data
between systems communicating at the same or different clock
frequencies.
Figure 5. Single Clock FIFO
Figure 6. Dual Clock FIFO
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ROM Configuration
The ROM configuration is shown in Figure 7 and has the following
features:
■
■
■
The memory contents are written during configuration before the
device enters user mode.
Allows registered or bypassed outputs.
The read operation is identical to the single-port RAM.
Figure 7. ROM Memory Configuration in an ESB
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ESB Read/Write Combinations
An ESB has two separate ports: A and B. Different modes use different
combination for these two ports. Table 2 shows read/write and
width/depth for different memory modes in the ESB. These events occur
within one clock cycle.
Table 2. Read/Write & Width/Depth Combinations in an APEX II ESB
Mode
Port A
Port B
Bidirectional
Dual-port RAM (1)
read
read
read
write
write
read
write
write
write
read
read
write
Single-port RAM
read/write
-
Single-port RAM
(packed mode) (2)
write/read
read/write
ROM
read
-
FIFO
read
write
Dual-port RAM (1)
Size
4096 × 1
1024 × 4
512 × 8
4096 × 1
1024 × 4
512 × 8
256 × 16
4096 × 1
1024 × 4
512 × 8
256 × 16
2048 × 1
256 × 8
512 × 4
4096 × 1
1024 × 4
512 × 8
256 × 16
4096 × 1
1024 × 4
512 × 8
256 × 16
Note to Table 2:
(1)
(2)
6
Supports mismatch in input/output data widths.
Packed mode is when the ESB is split into two 2K single-port memory blocks.
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Using the
MegaWizard to
Implement
Memory
Configurations
This section describes how to use the MegaWizard in the Quartus II
software to implement memory configurations through megafunctions:
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■
■
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■
LPM_RAM_DQ
LPM_RAM_DP
LPM_RAM_DP+
LPM_FIFO
LPM_ROM
Selecting a Megafunction to Customize
Select and customize a megafunction in a design using the MegaWizard
Plug-In Manager. The following are the steps to select an instance of a
megafunction using the Quartus II software:
1.
Select MegaWizard Plug-In Manager (Tools menu).
2.
Turn on the Create a new custom megafunction variation option.
See Figure 8.
3.
Click Next.
Figure 8. MegaWizard Plug-In Manager [page 1]
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4.
Choose a megafunction from the storage folder in the Available
Megafunction list. See Figure 9.
Figure 9. MegaWizard Plug-In Manager [page 2a]
8
5.
Turn on the type of output file to create: AHDL, VHDL, or Verilog.
6.
Type in a file name for the function.
7.
Click Next.
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LPM_RAM_DQ Megafunction
The LPM_RAM_DQ is a single-port RAM function as shown in Figure 10
and has the following features:
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■
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■
Implements either synchronous or asynchronous single-port RAM
inclock can register ESB data and/or address inputs
A separate outclock can register output data
LE-based or ESB-based
Synchronous write operations into the memory block use the address[]
and data[] ports, triggered by the rising edge of inclock while the
write enable (we) port is enabled. The outclock port for the read
operation is optional.
For asynchronous operations, setup and hold times have to be valid with
respect to both edges of the write enable signal (we). The data and address
lines should not change while we is active.
Figure 10. Single-Port RAM Block Diagram
Instantiating the LPM_RAM_DQ Megafunction in a Design
Use the following steps to create an instance of the lpm_ram_dq function:
Altera Corporation
1.
Follow the instructions in the “Selecting a Megafunction to
Customize” section, choosing lpm_ram_dq from the storage folder
in the Available Megafunction list.
2.
Choose the data output and address bus widths using the How wide
should the ‘q’ output bus be? and How wide should the ‘address’
input bus be? lists. See Figure 11. Any change made is automatically
reflected in the symbol graphic.
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3.
Turn on the options for the ports to register in the Which ports
should be registered? section. If registered, inclock clocks the data
and address ports; while outclock clocks the output port. See
Figure 11.
Figure 11. MegaWizard Plug-In Manager - LPM_RAM_DQ [page 3 of 5]
4.
Turn on the Yes, use this file for the memory content data option to
initialize the memory block with a hexadecimal (Intel-format) file
(.hex) or a memory initialization file (.mif) file. See Figure 12.
or
Turn on the No, leave it blank option to initialize the memory block
to all zero. See Figure 12.
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Figure 12. MegaWizard Plug-In Manager - LPM_RAM_DQ [page 4 of 5]
5.
Click Finish to complete the procedure. See Figure 13.
Figure 13. MegaWizard Plug-In Manager - LPM_RAM_DQ [page 5 of 5] - Summary
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LPM_RAM_DP Megafunction
The LPM_RAM_DP is a dual-port RAM function as shown in Figure 14 and
has the following features:
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■
■
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■
Fully parameterized function
Allows simultaneous read and write access to memory cells
Can register any combination of EAB/ESB inputs
LE-based or EAB/ESB-based
Four clocking modes
–
Single clock
–
Shared clock
–
Separate clock
–
Asynchronous
Clocking Modes
The lpm_ram_dp has four modes: single clock, shared clock, separate
clock, and asynchronous. Use the clocking method section of the
MegaWizard Plug-in Manager interface to select the different modes.
Single Clock
In single clock mode, the read and write operations are synchronous to the
same clock.
Shared Clock
In shared clock mode, in addition to having the read and write operations
synchronous to the same clock, a separate clock is used for the output
port, q[]. This is also referred to as having separate input and output
clocks.
Separate Clock
In separate clock mode, there are two independent clocks, rdclock and
wrclock, for the read and write operations, respectively (See Figure 14).
Asynchronous
The asynchronous mode requires no clock. The write operation is
dependent on the wren signal. The read operation is dependent on the
rden signal. If not present, the rden is VCC by default.
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Figure 14. Dual-Port RAM Block Diagram
Instantiating the LPM_RAM_DP Megafunction in a Design
Use the following steps to create an instance of the lpm_ram_dp function:
Altera Corporation
1.
Follow the instructions in the “Selecting a Megafunction to
Customize” section, choosing lpm_ram_dp from the storage folder
in the Available Megafunction list. See Figure 15.
2.
Choose the data output and address bus widths using the How wide
should the ‘q’ output bus be? and How wide should the ‘address’
input bus be? lists. Any change made is automatically reflected in
the symbol graphic.
3.
Turn on a clocking method in the Which clocking method do you
want to use? section.
4.
Optionally, turn on the Create a ‘rden’ read enable signal option.
The rden signal is tied to VCC (active), if no signal is created. See
Figure 15.
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Figure 15. MegaWizard Plug-In Manager - Dual-Port RAM [page 3 of 6]
5.
Turn on the options for Which ports should be registered?. Click
More Options… , to register the ports individually. See Figure 16 on
page 15.
6.
Optionally, turn on Create one clock enable signal for each clock
signal. If using dual clocks, there may be two separate clock enable
signals.
1
7.
14
The Create an ‘aclr’ asynchronous clear for the registered
ports is option is unavailable. In order to enable this option
use the altdpram function.
Click Next.
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Figure 16. MegaWizard Plug-In Manager - Dual-Port RAM [page 4 of 6]
8.
Turn on the Yes, use this file for the memory content data option to
initialize the memory block with a hexadecimal (Intel-format) file
(.hex) or a memory initialization file (.mif) file. See Figure 17.
or
Turn on the No, leave it blank option to initialize the memory block
to all zero.
9.
Click Next to see a summary of the files that MegaWizard creates.
or
Click Finish to complete the process.
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Figure 17. MegaWizard Plug-In Manager - Dual-Port RAM [page 5of 6]
LPM_RAM_DP+ (Bidirectional Dual-Port RAM) Megafunction
The LPM_RAM_DP+ is the bidirectional dual-port RAM function and has
the following features:
■
■
■
■
■
Fully parameterized function
Capable of simultaneous read and write access to memory cells
Capable of registering any combination of ESB inputs
LE-based or ESB-based
Three clocking modes
–
Single clock
–
Separate clock
–
Asynchronous
Clocking Modes
Use the clocking method section of the MegaWizard Plug-in Manager
interface to select one of the three LPM_RAM_DP+ clocking modes. The
three clock modes are as follows:
Single Clock
In the single clock mode, the read and write operations are synchronous
to the same clock.
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Separate Clock
In the separate clock mode, there are two independent clocks, clock_a and
clock_b for port A and port B, respectively.
Asynchronous
The asynchronous mode requires no clock. The write operation is
dependent on the wren signal, but the read operation is independent of
wren and can be continuous regardless of the write operation.
Instantiating a Bidirectional Dual Port RAM in a Design
Use the following steps to create an instance of the lpm_ram_dp+
function:
Altera Corporation
1.
Follow the instructions in the “Selecting a Megafunction to
Customize” section, choosing lpm_ram_dp+ from the storage folder
in the Available Megafunction list.
2.
Select APEX II for the device family from the Create RAM for which
device family? list.
3.
Turn on the With two read/write ports option in the How will you
be using the dual port ram? section. This option is for the
bidirectional dual port RAM configuration.
4.
Turn on either the memory size option for As a number of words or
As a number of bits in the How do you want to specify the memory
size? section. See Figure 18.
5.
Click Next.
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Figure 18. MegaWizard Plug-In Manager - Dual-Port [page 1 of 7]
18
6.
Select How many words of memory?. See Figure 19.
7.
Select the width for the output port A from the Read/Write Ports
section.
8.
Click Next.
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Figure 19. MegaWizard Plug-In Manager - Dual-Port RAM [page 2 of 7]
9.
Turn on the Single clock option as the clocking method in the Which
clocking method do you want to use? section. If using
asynchronous mode, turn on No Clock. See Figure 20.
10. Click Next.
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Figure 20. MegaWizard Plug-In Manager - Dual-Port Ram [page 3 of 7]
11. Check Write input ports and Read output ports to be registered. See
Figure 21.
12. Click More Options.
13. Turn on all of the required options in the RAM Port Registers dialog
box.
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Figure 21. MegaWizard Plug-In Manager - Dual-Port RAM [page 5 of 7]
14. Turn on the Yes, use this file for the memory content data option to
initialize the memory block with a hexadecimal (Intel-format) file
(.hex) or a memory initialization file (.mif) file. See Figure 22.
or
Turn on the No, leave it blank option to initialize the memory block
to all zeros.
15. Click Next to see a summary of the files that MegaWizard creates.
or
Click Finish to complete the process.
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Figure 22. MegaWizard Plug-In Manager - Dual-port RAM [page 6 of 7]
LPM_FIFO+ Megafunction
The LPM_FIFO+ is a single- or dual-clock FIFO megafunction with the
following features:
■
■
■
■
■
■
Fully parameterized function
Single- or dual-clock modes
Simultaneous reads and writes
Full, empty, and used words output signals
Fully optimized for Altera architecture
LE-based or EAB/ESB-based
The lpm_fifo+ function is a parameterized, single or dual clock FIFO
megafunction that can be used to buffer data between systems
communicating at the same or different clock frequencies. The
lpm_fifo+ function is actually two functions in one – a single clock FIFO
(lpm_fifo) and a dual clock FIFO (lpm_fifo_dc). Select which FIFO
megafunction to create in the MegaWizard PlugIn Manager interface.
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Single Clock Mode
In single clock mode, the lpm_fifo+ megafunction instantiates the
scfifo megafunction for single clock FIFO functions. The megafunction
has a single clock port for writing and reading data to and from the FIFO.
Simultaneous reads and writes can be done in the same clock cycle.
The parameterization of the lpm_fifo+ megafunction can implement
any width/depth combination. The function also has customizable output
signals, such as empty/full flags and number of used words outputs. It
also gives the choice of an asynchronous or a synchronous clear signal.
Either ESBs or LEs can implement the megafunction. However,
implementing a large FIFO in LEs can dramatically increase overall LE
use.
Dual Clock Mode
In dual clock mode, the lpm_fifo+ megafunction instantiates the
dcfifo megafunction for dual-clock FIFO functions. The megafunction
has two independent clock ports used for writing and reading data to and
from the FIFO buffer. Simultaneous reads and writes can be done in the
same clock cycle.
The parameterization of the lpm_fifo+ megafunction can implement
any width/depth combination. The function also has customizable output
signals, such as empty/full flags and number of used words outputs, that
can be synchronized to either the read or write clock.
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Altera Corporation
The last three words of the dcfifo may not be available for
writing because of the synchronization pipelines between the
two clock domains. These pipelines are intended to avoid
internal metastability. Because of these pipelines, information
available to one clock domain regarding when read and write
conditions occur may be temporarily unavailable to the other
clock. The wrfull and rdfull ports of an lpm_fifo+
megafunction must go high slightly before the dcfifo is
completely full, in order to avoid overshooting the top of the
FIFO. This process may cause several words at the end of the
FIFO to become unavailable. Depending on the write rate to the
FIFO, the wrfull and rdfull ports may go high with three
words remaining, with two words remaining, or with one word
remaining in the FIFO. However, this process is necessary both
to accommodate the clock synchronization and to ensure overflow does not take place. To maintain a specific number of
words, specify a number for the LPM_NUMWORDS parameter that
is up to three words greater than the amount needed.
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The synchronization and internal logic in dcfifo may delay the
information in the wrempty, wrfull, wrusedw[], rdempty,
rdfull, and rdusedw[] ports by one clock cycle of latency.
Setup and hold violations on synchronization registers during
timing analysis can occur if the rdclock and wrclock signals
are unrelated. This is normal behavior for multi-clock designs.
Common to Both Dual & Single Clock Modes
The read-request port of the lpm_fifo+ megafunction has two
configuration modes: legacy synchronous FIFO and show-ahead
synchronous FIFO. These modes configure the read-request port to act as
either a read request or a read acknowledge.
In the default read request (legacy) mode, requested data comes out of the
FIFO on the first clock cycle after the read request is asserted.
In the read acknowledge (show-ahead) mode, the first piece of data
written into the FIFO immediately shows up on its output and reads from
the FIFO are acknowledged instead of requested (See Figure 23).
Figure 23. FIFO in an ESB
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Instantiating the LPM_FIFO+ Megafunction in a Design
This section list the steps needed to create an instance of the lpm_fifo+
megafunction in single-clock and in dual-clock mode.
Single-Clock FIFO
Use the following steps to create an instance of the lpm_fifo+
megafunction in single-clock mode:
1.
Follow the instructions in the “Selecting a Megafunction to
Customize” section, choosing lpm_fifo+ from the storage folder in
the Available Megafunction list.
2.
Select the number of bits for How wide should the FIFO be? and the
number of words for How deep should the FIFO be?. See Figure 24.
3.
Turn on the Yes, synchronize both reading and writing to ‘clock’
option.
4.
Click Next.
Figure 24. MegaWizard Plug-In Manager - LPM_FIFO [page 3 of 8]
Altera Corporation
5.
Turn on the desired options in the Which optional output control
signals do you want? section. See Figure 25.
6.
Click Next.
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Figure 25. MegaWizard Plug-In Manager - LPM_FIFO [page 4 of 8]
7.
Turn on which rdreq signal mode (legacy synchronous or showahead synchronous) to use in the Which kind of read access do you
want with the ‘rdreq’ signal? section. See Figure 26.
8.
Click Next.
Figure 26. MegaWizard Plug-In Manager - LPM_FIFO [page 6 of 8]
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9.
Turn on the Quartus II software optimization (default, speed, or
area) to use with the megafunction in the Which type of
optimization do you want? section.
10. Optionally, turn on Disable overflow checking and/or Disable
underflow checking to disable circuitry protection.
11. Optionally, turn on the Implement FIFO function with logic cells
only, even if the device contains EABs or ESBs compiler option.
The default is to place the FIFO in ESBs. See Figure 27.
12. Click Next to see a summary of the files that MegaWizard creates.
or
Click Finish to complete the process.
Figure 27. MegaWizard Plug-In Manager - LPM_FIFO [page 7of 8]
Dual-Clock FIFO
Use the following steps to create an instance of the lpm_fifo
megafunction in dual-clock mode:
1.
Altera Corporation
Follow the instructions in the “Selecting a Megafunction to
Customize” section, choosing lpm_fifo from the storage folder in
the Available Megafunction list.
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2.
Select the number of bits for How wide should the FIFO be? and the
number of words for How deep should the FIFO be?.
3.
Turn on No, synchronize reading and writing to ‘rdclock’ and
‘wrclock’, respectively”. See Figure 28.
4.
Turn on the Yes option to add latency or the No option to not add
latency in the Do you want the FIFO to pipeline its output flags?
section.
5.
Click Next.
Figure 28. MegaWizard Plug-In Manager - LPM_FIFO [page 3 of 8]
28
6.
Turn on the required read-side and write-side Which optional
output control signals do you want? options. Each of the full,
empty or usedw[] signals can be synchronized to either the read or
write clock. See Figure 29
7.
If required, turn on the Asynchronous clear. See Figure 29.
8.
Click Next.
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Figure 29. MegaWizard Plug-In Manager - LPM_FIFO [page 5 of 8]
9.
Turn on which rdreq signal mode (legacy synchronous or showahead synchronous) to use in the Which kind of read access do you
want with the ‘rdreq’ signal? section. See Figure 30.
10. Click Next.
Figure 30. MegaWizard Plug-In Manager - LPM_FIFO [page 6 of 8]
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11. Turn on the desired optimization type in the Which type of
optimization do you want? section.
12. Optionally, turn on whether you want to disable overflow or
underflow checking in the Would you like to disable any circuitry
protection? section.
13. Optionally, turn on the Implement FIFO function with logic cells
only, even if the device contains EABs or ESBs compiler option.
The default is to place the FIFO in ESBs. See Figure 31.
Figure 31. MegaWizard Plug-In Manager - LPM_FIFO [page 7of 8]
14. Click Next to see a summary of the files that MegaWizard creates.
or
Click Finish to complete the process.
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LPM_ROM Megafunction
The LPM_ROM is a ROM function that has the following features:
■
■
■
■
Fully parameterized function
Supports synchronous and asynchronous memory modes
Allows control of the address input and ROM output with separate
clocks
Fully optimized for Altera architecture
The parameterized lpm_rom function can implement all ROM functions.
The ESB implement these functions in APEX II devices.
The lpm_rom megafunction supports both synchronous and
asynchronous modes of operation. The address[] input port and/or the
q[] output port can be registered and are controlled by the inclock and
outclock, inputs respectively. Totally asynchronous memory
operations occur when both the inclock and outclock ports are unused.
Further, the lpm_rom megafunction provides the optional use of a
memory enable signal – the memenab port. When the memory is not
enabled (memenab is low), the q[] output is high-impedance (See
Figure 32).
1
lpm_rom must have a memory initialization file (.mif) or
hexadecimal (.hex) file with the same name in the project
directory that contains the data written to ROM during
configuration. For more information on creating these files see
“Initialization Files” on page 55.
Figure 32. ROM Block Diagram
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Instantiating the LPM_ROM Megafunction in a Design
Use the following steps to create an instance of the lpm_rom
megafunction in single-clock mode:
1.
Follow the instructions in the “Selecting a Megafunction to
Customize” section, choosing lpm_rom from the storage folder in
the Available Megafunction list.
2.
Specify the size of the ROM by selecting the number of bits in the
How wide should the ‘q’ output bus be? section and the number of
bits in the How wide should the ‘address’ input bus be? section.
3.
For synchronous ROM implementation, in the Which ports should
be registered? section, turn on the ‘address’ input port option
and/or the ‘q’ output port option. See Figure 33.
4.
Click Next.
Figure 33. MegaWizard Plug-In Manager - LPM_ROM [page 1 of 3]
5.
32
To initialize the memory block with a hexadecimal (Intel-format) file
(.hex) or a memory initialization file (.mif) file, browse to the file
location in the What is the name of the file containing the memory
initialization data? section. See Figure 34.
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6.
Click Next to see a summary of the files that MegaWizard creates.
or
Click Finish to complete the process.
Figure 34. MegaWizard Plug-In Manager - LPM_ROM [page 2 of 3]
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ESB Timing
Diagrams
Figures 35 - 72 show the timing diagram for different ESB modes, such as
single-port, dual-port, bidirectional dual-port RAM and ROM.
Single-Port RAM
Synchronous write operations into the memory block uses the
address[] and data[] port, triggered by the rising edge of inclock
while the write enable (we) port is enabled. The outclock port for the
read operation is optional. The largest usage of dual-port RAMs is in
communications, which includes the exchange of data between
processors and systems.
Figures 35 - 40 show the synchronous and asynchronous timing diagrams
of a single-port RAM.
Figure 35. Synchronous Read/Write Cycle Timing of a Single-Port RAM (Unregistered Output))
Figure 36. Synchronous Read/Write Block Diagram of a Single-Port RAM (Unregistered Output)
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Figure 37. Synchronous Read/Write Cycle Timing of a Single-Port RAM (Registered Output)
Figure 38. Synchronous Read/Write Block Diagram of a Single-Port RAM (Registered Output)
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Figure 39. Asynchronous Read/Write Cycle Timing of a Single-Port RAM Note (1)
Note to Figure 39:
(1)
In asynchronous mode, when we (write enable) is asserted, the address cannot change.
Figure 40. Asynchronous Read/Write Block Diagram of a Single-Port RAM
Dual-Port RAM
Figures 41 - 52 show the dual-port RAM read/write configurations and
timing diagrams.
Read/Write
The following situations can occur when simultaneously reading and
writing to the same address in dual-port RAM:
■
Unregistered output port
1.
36
The read clock's frequency is greater than 2x the write clock's
frequency. The write clock has not written the data at this point;
therefore, the old data value is read.
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2.
Dual-port RAM uses the same clock for reading and writing.
The newly written data appears at the output after a delay of
tESBDD after the falling edge of the clock. At slower clock
frequencies, the old data value can appear shortly after the
rising edge of the clock followed by the newly written data,
which appears tESBDD nanoseconds after the falling edge.
1
■
Check the data sheet for the value of tESBDD in the timing
model section.
Registered output port
3.
The read clock is very fast (frequency > 1/tEABDD). The q output
reads the old data value.
4.
Dual-port RAM uses the same clock for reading and writing.
The q output reads the newly written data value.
5.
The read and write clocks are unrelated, and the read clock has a
frequency less than 1/tEABDD. The q output reads the newly
written data value.
Figure 41. Dual-Port RAM Read/Write Cycle Timing in Single Clock Mode (Unregistered Output)
Note to Figure 41:
(1)
If the read and write operations are targeting the same memory location, the newly written data will be available
on the output port q[] on the falling edge of the clock.
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1
When a read occurs to the same address location with a write
operations in progress (write not completed), there is a
possibility for having unknown output from the ESB. To prevent
this potential contention, the read operation should not start
until the write operation is completed. For this to occur, the read
operation should not be activated for a minimum amount of time
specified as the maximum write cycle time in the ESB. This
parameter is tESBSRC and is specified in the data sheet in the
timing model section for each device.
Figure 42. Dual-Port RAM Read/Write Block Diagram in Single Clock Mode (Unregistered Output)
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Figure 43. Dual-Port RAM Read/Write Cycle Timing in Single Clock Mode (Registered Output)
Figure 44. Dual-Port RAM Read/Write Block Diagram in Single Clock Mode (Registered Output)
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Figure 45. Dual-Port RAM Read/Write Cycle Timing in Shared Clock Mode (Registered Output)
Notes to Figure 45:
(1)
(2)
(3)
If both read and write operations are targeting the same memory location, the present read cycle will result in data
that was previously written into that memory location. The newly written data is only present in the next read cycle.
rden is deasserted so that the newly written data into 0A is valid on the output port q[].
New data at memory location 0A is not driven out on the output port q[] because the next read cycle is retrieving
data from the previous memory location 0A.
Figure 46. Dual-Port RAM Read/Write Block Diagram in Shared Clock Mode (Registered Output)
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Figure 47. Dual-Port RAM Read/Write Cycle Timing in Separate Clock Mode (Registered Output)
Note to Figure 47
(1)
During this simultaneous read/write operation the new data is written in memory location 0B at the falling edge
of wrclock. Since the output port q[] is registered, this newly written data, “2” is available at the output port q[]
only during the next rdclock cycle.
Figure 48. Dual-Port RAM Read/Write Block Diagram in Separate Clock Mode (Registered Output)
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Figure 49. Dual-Port RAM Read/Write Cycle Timing in Separate Clock Mode (Unregistered Output)
Note to Figure 49:
(1)
Since the output port q[] is unregistered, the newly written data at memory location 0B is available at the falling
edge of wrclock.
Figure 50. Dual-Port RAM Read/Write Block Diagram in Separate Clock Mode (Unregistered Output)
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Figure 51. Dual-Port RAM Read/Write Cycle Timing in Asynchronous Clock Mode Note (1)
Note to Figure 51:
(1)
In asynchronous mode, when we (write enable) is asserted, the address cannot be changed.
Figure 52. Dual-Port RAM Read/Write Block Diagram in Asynchronous Clock Mode
Bidirectional Dual-Port RAM
Figures 53 - 60 show the bidirectional dual-port RAM read and write
operations on both ports A and B.
Read/ Write
In bidirectional dual-port RAM, each port can read and write. As a result,
port A and port B can write at the same time, read at the same time, or read
and write alternately. It might happen that both ports write to the same
address location at the same time, which causes an invalid data. To
prevent this potential write contention, make sure that writes do not occur
at the same address simultaneously.
Single Clock Mode
Only one clock is used for the input, output, write, and read signals. The
output is available one clock cycle later because the inputs and outputs are
registered.
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Figure 53. Bidirectional Dual-Port RAM Write & Read Cycle TIming in Single Clock Mode (Inputs & Outputs
Registered)
Figure 54. Bidirectional Dual-Port RAM Write & Read Block Diagram in Single Clock Mode (Inputs & Outputs
Registered)
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Figure 55. Bidirectional Dual-Port RAM Write & Read Cycle Timing in Single Clock Mode (Only Inputs
Registered)
Figure 56. Bidirectional Dual-Port RAM Write & Read in Single Clock Mode (Only Inputs Registered)
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Double Clock Mode
Each port can have a separate clock. The same clock performs writes and
reads on each port. See Figures 57 - 58.
Figure 57. Bidirectional Dual-Port Write & Read Cycle Timing RAM Dual Clock Mode (Inputs & Outputs
Registered)
Figure 58. Bidirectional Dual-Port RAM Write & Read Block Diagram Dual Clock Mode (Inputs & Outputs
Registered)
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Asynchronous Bidirectional Dual-Port RAM
Figures 59 - 60 show the asynchronous bidirectional dual-port RAM
timing diagram.
Figure 59. Asynchronous Bidirectional Dual-Port RAM Cycle Timing with Simultaneous Write & Read on
Ports A & B
Notes to Figure 59:
(1)
(2)
(3)
Data 6, 7, and 0 is written into address 3.
Output port B reads out data 6, 7, and 0.
Write enable for port A and B should be high to write or read from memory and should stay low during address
transition.
Figure 60. Asynchronous Bidirectional Dual-Port RAM Block Diagram with Simultaneous Write & Read on
Ports A & B
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FIFO
Figures 61 - 67 show the read/write cycle of a single or double clock FIFO.
Figure 61. Single-Clock FIFO: Write, Empty Flag, & Simultaneous Read/Write Cycle Waveform (Legacy
Synchronous FIFO Mode)
Figure 62. Single-Clock FIFO Full Flag Waveform (Legacy Synchronous FIFO Mode)
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Figure 63. Single-Clock FIFO Full Flag Waveform (Legacy Synchronous FIFO Mode)
Figure 64. Single-Clock FIFO: Read Cycle & Empty Flag (Legacy Synchronous FIFO Mode)
Note to Figure 64:
(1)
There are six clock cycles of latency on the rdempty flag. This number can be changed by changing the delay
parameters listed earlier.
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Figure 65. Dual-Clock FIFO Read Cycle Waveform (Legacy Synchronous FIFO Mode)
Note to Figure 65:
(1)
There are six clock cycles from the first rdclock edge after the data is written to when the data is available to be
read. This number can be changed by changing the delay parameters. Also, note that the rdempty flag went high
after the second read was done (reading 2). This is due to the relationship between the read an write frequencies.
Figure 66. Dual-Clock FIFO Full Flag Waveform (Legacy Synchronous FIFO Mode)
Note to Figure 66:
(1)
In this case, the FIFO is 32 words deep. Note that when the 31st word is written into the FIFO, the wfull flag goes
high. For an explanation, please see “Dual Clock Mode” on page 23.
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Figure 67. Dual-Clock FIFO Read-Acknowledge Waveform (Show-Ahead Mode)
Note to Figure 67:
(1)
The first piece of data that is written into the FIFO flows through to the output on the next clock edge of rdclock.
In this type of FIFO, the read is acknowledged and not requested. Therefore, even though data “3” is on the q[] port
after the second read is done, it still has not been read.
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ROM
Figures 68 - 73 show the operation of the lpm_rom function in the
following modes
■
■
■
Asynchronous
Synchronous with q[] output registered
Synchronous with address[] and q[] ports registered
Figure 68. Read Timing During Asynchronous Memory Operation
Note: to Figure 68:
(1)
During this asynchronous memory operation, the data on the q[] output changes after a certain delay following a
change in the address[] input. The MIF file used by this ROM function specifies the data in locations FF and 33 to
be FFFF and 3333, respectively.
Figure 69. Read Timing During Asynchronous Memory Operation
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Figure 70. Read Timing With Registered ROM Output q[]
Note: to Figure 70:
(1)
When the q[] output port is registered, the output register is loaded with the contents of the memory location
pointed to by address[] at every rising edge of outclock.
Figure 71. Read Timing With Registered ROM Output [q]
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Figure 72. Read Timing With Registered address[] Input & q[] Output Using The Same Clock
Note to Figure 72:
(1)
The memenab port tri-states the q[] output when it has a logic low level. Although, the memenab port does not
affect the address[] input from being registered. After memenab is set to a logic high level, the q[] output register
captures the data from the ROM location pointed to by the registered address[] input.
Figure 73. Read Timing With Registered address[] Input & q[] Output Using The Same Clock
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Initialization
Files
A memory initialization file (MIF) or hexadecimal file (HEX) specifies the
initial value for each memory address.
Memory Initialization File
An ASCII text file with the extension .mif specifies the initial content of a
memory block. These initial values for each address in the .mif file are
used during project compilation and/or simulation. Use the Memory
Editor in the Quartus II software to create .mif files.
Each memory block requires a separate .mif file. A MIF must also specify
the memory depth and width values.
Optionally, a MIF can specify the radixes used to display and interpret
addresses and data values.
1
If multiple values are specified for the same address, only the last
value is used.
Figure 74 shows a sample .mif file.
Figure 74. Sample MIF File
DEPTH = 32;
WIDTH = 14;
ADDRESS_RADIX = HEX;
DATA_RADIX = HEX;
% Memory depth and width are required. %
% Enter a decimal number. %
% Address and value radixes are required. %
% Enter BIN, DEC, HEX, OCT, or UNS unless otherwise %
% specified, radixes = HEX. %
Specify values for addresses, which can be a single address or range.
CONTENT
BEGINS
[0..F] : 3FFFF;
6 : F;
8 : F E 5;
%
%
%
%
Range--Every address from 0 to F = 3FFFF. %
Single address. Address 6 = F. %
Range starting from specific address. %
Addr[8] = F, Addr[9] = E, Addr[A] = 5. %
END
HEX (Intel-Format) File
The Quartus II software can use an ASCII text file with the extension .hex
in the Intel-format to store configuration data for one or more Altera
devices, to store the initial memory values for a memory block
implemented in an Altera device, or to build software project executables.
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A HEX file can be used as an input file for memory initialization in the
Compiler and Simulator. Use the Memory Editor to create a HEX File.
The Software mode can use a HEX file as an output file. A software
application can be built as a HEX File.
Creating a MIF or HEX File
To create a (.mif) or (.hex):
1.
Choose New (File menu).
2.
In the New dialog box, click the Other Files tab.
3.
To create a MIF, select Memory Initialization File or to create a HEX
File, select Hexadecimal File.
4.
Click OK.
5.
In the Number of Words & Word Size dialog box, type the number
of words in the Number of words box.
6.
In the Word size box, type the size of the words.
7.
Click OK.
Figure 75 shows a sample MIF table.
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Figure 75. An 8-bit MIF Table
Applications of
ESB Memory
APEX II ESB memory configurations are suitable for a variety of
applications. The following paragraphs suggest possible applications of
the different memory configurations.
RAM Configurations
RAM memory can be used for communication between systems. In these
applications, RAM can be partitioned into virtual data-storage areas, or
buffers, usually storing at least two data packets. The buffers are shared
between the communications controller and the intelligent host that
assembles the packets and stores them—usually a microprocessor. If the
system contains only one processor, the data buffers are not shared and
the system needs neither a virtual nor a physical dual-port RAM.
Single-Port RAM
Single-port RAM can be used in applications that require parallel data
transfer because the write and read ports are the same width.
Dual-Port RAM Configuration
Dual-port RAM has applications in the general areas of wide area
networks, storage networks, and wireless infrastructures. Some specific
applications can be in switches and routes, RAID directors, host bus
adapters, and cellular base stations.
Dual-port RAM can be used in applications that require parallel data
transfer, because the two independent clock ports allow different access
rates for read and write operations. Also, the presence of dual-addressing
enables simultaneous read and write operations in the same clock cycle.
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Bus Width Mismatch
A simple dual-port RAM can be used in APEX II devices for read and
write width mismatch since RAM cells have independent read and write
ports. This feature can be used for serialization or deserialization of some
high-speed applications that use ESB blocks.
Because the speed of some signals to the ESB may be too high to b used for
internal logic, they need to be slowed down. Deserializing the serialized
incoming signals is the method used to slow down signals. The ESB’s
dual-port RAM feature can perform deserialization and serialization
because it supports a mismatched bus width configuration on its ports.
Bidirectional Dual-Port RAM
Because bidirectional dual-port RAM can read or write simultaneously at
both ports, it can be used in applications that require quick data access,
such as in switches and routers.
ROM Configuration
ROM can be used to implement all functions that only need to perform
read operations.
FIFO Configuration
Using FIFO is a way to increase data throughput to an I/O interface while
minimizing central processing unit (CPU) wait states. FIFO buffers
transfer data between the host bus and the peripheral systems. Some
common applications using FIFO buffers are SCSI and integrated drive
electronics (IDE) interfaces, bus-width conversion applications (e.g., 8-bit
to 32-bit conversions), and asynchronous transfer mode (ATM) network
interface cards.
Conclusion
58
With bidirectional dual-port RAM becoming the desired and preferred
choice in the market, the ESBs in APEX II devices have been enhanced to
accommodate this new memory structure. Bidirectional dual-port RAM
can be used in different applications where the ability to access data
simultaneously by two different processes is a requirement. In addition,
the ESB in APEX II with its enhanced memory structure is able to divide
the ESB into two identical 2K single-port RAM blocks for applications that
required more memory blocks.
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101 Innovation Drive
San Jose, CA 95134
(408) 544-7000
http://www.altera.com
Applications Hotline:
(800) 800-EPLD
Literature Services:
[email protected]
59
Copyright © 2002 Altera Corporation. All rights reserved. Altera, The Programmable Solutions Company, the
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