When Timing-Is-Everything …
Charge Transfer Model for
Input Signaling & Referencing
Bill Gervasi
Technology Analyst
[email protected]
([email protected])
Mark Sherwood and Associates
All rights reserved
Copyright JANUARY, 2003
When Timing-Is-Everything …
Agenda
•
•
•
•
•
•
Review traditional setup & hold time
Examine impact of signal slew rate
Compare single ended to differential
What about non-monotonic signals?
Looking to the future
Call to action
Mark Sherwood and Associates
All rights reserved
Copyright JANUARY, 2003
When Timing-Is-Everything …
Why a New Model?
• Variable Frequency systems expanding
– Fixed frequency systems just needed a
yes/no answer on signal integrity
– Variable frequency systems need more:
calculated based on available data eye
Available operating frequency =
1  (required data eye + crap)
Mark Sherwood and Associates
All rights reserved
Copyright JANUARY, 2003
When Timing-Is-Everything …
Traditional Input Timing Model
CLOCK
CLOCK
Setup
Hold
INPUT
Timing derating by input signal slew rate:
1.0V/ns = base value
0.5V/ns = base value + 50ps
0.4V/ns = base value + 100ps
This got us through 266-333 MHz data rates…
Mark Sherwood and Associates
All rights reserved
Copyright JANUARY, 2003
When Timing-Is-Everything …
Problems with Traditional Model
• Does not account for differences between
single ended and differential
• Assumes setup & hold remain balanced
• No analytical way to project derating
• No guidance to package developers
• No mechanism to handle non-monotonic
signal transitions
• Leaves important picoseconds wasted
Mark Sherwood and Associates
All rights reserved
Copyright JANUARY, 2003
When Timing-Is-Everything …
Charge Transfer Model (CTM)
INPUT
tEXT
tINT
tT
Charge Transfer Timing Model
– All inputs have a slew rate dependent aspect tEXT
and an independent aspect tINT
– Summing tEXT + tINT gives input transition time tT
– Transition time tT has min and max values
– Differential input transitions inherently different
Mark Sherwood and Associates
All rights reserved
Copyright JANUARY, 2003
When Timing-Is-Everything …
Factors Affecting Transition
• External factors that are edge rate
dependent
– Package parasitics
– Input stage 1 characteristics including
input type & saturation
• Internal factors that are edge rate
independent
– Internal loading mismatch
– On-chip routing
– Process, voltage, temperature
Mark Sherwood and Associates
All rights reserved
Copyright JANUARY, 2003
When Timing-Is-Everything …
Standards Versus Proprietary
• CTM was developed inside JEDEC for DDR2
• Standards require an envelope within which multiple
suppliers specs operate, therefore:
–
–
–
–
I use VREF as an accumulation start threshold
I use VILAC and VIHAC as saturation thresholds
These do not represent all input types, but
They do define a standard way to define the envelope
• Proprietary systems analyses could use real
thresholds for increased application specific accuracy
Mark Sherwood and Associates
All rights reserved
Copyright JANUARY, 2003
When Timing-Is-Everything …
tEXT for Slow Slew Rate, Single Ended, simple model
VIHAC =
VREF + VSAT
AT = Charge
to Transition
VIHDC
VREF
tEXT
tINT
VILDC
tT
VILAC =
VREF - VSAT
Mark Sherwood and Associates
t EXT 
All rights reserved
2 * AT
slew
Copyright JANUARY, 2003
When Timing-Is-Everything …
tEXT for Fast Slew Rate, Single Ended, simple model
AT = ASAT + AADD
VSAT = Saturation
Voltage Offset
AADD = Charge
after Saturation
VIHAC =
VREF + VSAT
VIHDC
ASAT = Charge
to Saturation
VREF
tSAT
VILDC
tEXT
tINT
tT
VILAC =
VREF - VSAT
Mark Sherwood and Associates
t EXT  t SAT 
( AT  ASAT )
VSAT
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Internal factors
Copyright JANUARY, 2003
When Timing-Is-Everything …
tEXT for Slow Slew Rate, Differential, simple model
VIHAC =
VREF + VSAT
AT = Charge
to Transition
VIHDC
VREF
VILDC
VILAC =
VREF - VSAT
tINT
tEXT
tT
Mark Sherwood and Associates
All rights reserved
Copyright JANUARY, 2003
When Timing-Is-Everything …
tEXT for Fast Slew Rate, Differential, simple model
AT = ASAT + AADD
VIHAC =
VREF + VSAT
VIHDC
VREF
VILDC
VILAC =
VREF - VSAT
tEXT
tINT
tT
Mark Sherwood and Associates
All rights reserved
Copyright JANUARY, 2003
When Timing-Is-Everything …
Applying CTM to Setup & Hold
• Relate input signals (data, address,
command) to reference (clock, strobe)
• Assert tTmax and tTmin characteristics
• Assume inputs and references may have
different input slew rates
• Outcome is a matrix of deratings
Mark Sherwood and Associates
All rights reserved
Copyright JANUARY, 2003
When Timing-Is-Everything …
CTM Setup Time Definition
CLOCK
tTmin
CLOCK
Setup
INPUT
tTmax
Setup = tTmax of input – tTmin of reference
Mark Sherwood and Associates
All rights reserved
Copyright JANUARY, 2003
When Timing-Is-Everything …
CTM Hold Time Definition
CLOCK
tTmax
CLOCK
Hold
tTmin
INPUT
Hold = tTmax of reference – tTmin of input
Mark Sherwood and Associates
All rights reserved
Copyright JANUARY, 2003
When Timing-Is-Everything …
How does this help…?
The Charge Transfer Model gives a higher accuracy for
setup and hold relationships
It also provides a way to accurately describe derating
for input slew rate
Charge Transfer Model gives a method for analyzing
non-monotonic transitions
Mark Sherwood and Associates
All rights reserved
Copyright JANUARY, 2003
When Timing-Is-Everything …
Example: DDR2 Input Derating Tables
Strobe (mV/ps avg)
0.5
1.0
2.0
0.5
+
+
1.0

0
+
+
2.0



SETUP
Addr (mV/ps)
Data (mV/ps)
SETUP
0.5
1.0
2.0
0.5
+
+
1.0

0
+
+
2.0



Strobe (mV/ps avg)
HOLD
0.5
1.0
2.0
+
+
1.0
+
+
0

2.0



0.5
Mark Sherwood and Associates
Addr (mV/ps)
Data (mV/ps)
HOLD
All rights reserved
Clock (mV/ps avg)
Clock (mV/ps avg)
0.5
1.0
2.0
+
+
1.0
+
+
0

2.0



0.5
Copyright JANUARY, 2003
When Timing-Is-Everything …
Derating Using Charge Transfer
• Users continue to use VREF crossing for single ended
and crosspoint of differential pair for measurement
• Derating tables correlate measurement to required
setup & hold times
• “Boot-time” slew rate calculations may be needed,
e.g. modules stuffed and total loading
The Charge Transfer Model reduces system
development cost by enabling more complex timing
analysis
Mark Sherwood and Associates
All rights reserved
Copyright JANUARY, 2003
When Timing-Is-Everything …
Non-Monotonic Transitions
• Line reflections in complex system loading or enduser configurable systems cause shelves or ringback,
for example…
Memory Slots
Data bus
1-4 loads per slot
Memory
Controller
Address/Command bus
4-36 loads per slot
Clocks
Fixed loading per slot
Mark Sherwood and Associates
All rights reserved
Copyright JANUARY, 2003
When Timing-Is-Everything …
Charge Transfer Model for Non-Monotonic Transitions
Ringback here
should not matter
Accumulated charge
reaches mV-ps needed to
trigger input
VIHAC =
VREF + VSAT
VIHDC
VREF
tINT
tEXT
VILDC
tT
VILAC=
VREF - VSAT
Mark Sherwood and Associates
All rights reserved
Copyright JANUARY, 2003
When Timing-Is-Everything …
Ringback below accumulation threshold
A3
A1
A2
VIHAC =
VREF + VSAT
VIHDC
VREF
VILDC
tINT
tEXT
tT
VILAC=
VREF - VSAT
Mark Sherwood and Associates
Charge Accumulation =
A1 – A2 + A3
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Copyright JANUARY, 2003
When Timing-Is-Everything …
Next Generation Models
• Characterize and combine input types
– Where tEXT is the same and only tINT is different
•
•
•
•
•
More complex geometrical area calculations
More accuracy in accumulation & saturation points
Non-linearities in charge accumulation
Model for ground bounce
Model for power supply noise
Mark Sherwood and Associates
All rights reserved
Copyright JANUARY, 2003
When Timing-Is-Everything …
Summary
• Charge Transfer Model separates input signal
characteristics
– Slew rate dependent
– Slew rate independent
• Single ended and differential derate
differently
• Matrix derating tables needed to handle CTM
• Non-monotonics can be dealt with
Mark Sherwood and Associates
All rights reserved
Copyright JANUARY, 2003
When Timing-Is-Everything …
Call to Action
• Extend IBIS models with CTM
• Need logic analysis tools for calculating
charge accumulation
– Accumulation threshold
– Saturation threshold
– Comprehend setup & hold timing derating
• mV-ps accumulation area
• Line up or mark tT(min) & tT(max)
• Single ended versus differential accumulation
• Looking ahead
– Algorithmic accumulation model
Mark Sherwood and Associates
All rights reserved
Copyright JANUARY, 2003
When Timing-Is-Everything …
Thank You
Mark Sherwood and Associates
All rights reserved
Copyright JANUARY, 2003