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AMS Assertions
… beyond analog waveform
checking
Mike Demler
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Categories of AMS properties
1. Connectivity properties
2. Static electrical properties
3. Functional properties
4. Signal properties I:
–
Single-event temporal properties
5. Signal properties II:
–
Cumulative temporal properties
6. Signal properties III:
–
Frequency domain properties
7. Mixed-signal interface properties
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Categories covered in initial proposal
1.
Connectivity properties
2.
Static electrical properties
3.
Functional properties
4.
Signal properties I:
–
Single-event temporal properties
5.
Signal properties II:
– Cumulative temporal properties
6.
Signal properties III:
– Frequency domain properties
7.
Mixed-signal interface properties
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A Few Examples
Analog properties… that waveform checks miss
• One of the most critical applications for mixedsignal verification is in SoC power management.
• Functional verification of digitally-controlled
power modes
– How to verify consumption meets budget?
– Integration of AMS IP
• Is the power supply a signal?
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Analog properties
… that waveform checks miss.
• PROBLEM:
– Multiple power domains. Low voltage
inverter mistakenly connected to higher
voltage drivers, “digitally” switched.
• Output voltage waveform appears to
be functionally correct.
2.5V
– assert property (V(G)<=1.2); ?
• Requires device-level terminal
property monitor.
n1
1.2V
(0,2.5V) Max=4n3
n7
n2
(0,2.5V) Max=4
1.2V
2.5V
n4
n6
n5
(0,2.5V) Max=4
Vgs(Minimum, Maximum) NMOS
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n8
Analog properties
… that waveform checks miss.
• PROBLEM:
– Power down results in unexpectedly high power consumption.
• Output waveform appears to be correct. Long time domain.
• Requires device-level “X” (or high-Z) property monitor.
3.0
3.0
3.0
PDWN
3.0
PDWN
PDWN
3.0
3.0
3.0
3.0
0.0
3.0
3.0
Hi-Z(0.0)
3.0
Power Down
Seconds/Minutes
_PDWN
_PDWN
_PDWN
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Analog properties
… that waveform checks miss.
• PROBLEM:
– DC paths result in wasted power.
• Output voltage signals looks OK.
• How to monitor supplies, DC terminal currents?
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AMS Assertions
• Automation of time-domain signal analysis is
valuable, but only covers a subset of important
analog properties.
• The waveform is not the circuit.
– …and it is the circuit that must be verified.
• Just as V-AMS LRM added SPICE constructs…
– AMS assertions must also understand devices and
circuits to provide adequate verification coverage.
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AMS Assertions
• Previous examples are the most frequent
causes of AMS SoC failures.
• Recent customer quote:
– “The following issues have caused silicon failures on
our mixed-signal chips: floating gates, high voltage on
single oxide transistors, level-shifter insertion…”
• Failures occur most often because of the signal
that wasn’t simulated!
– Rarely because a simulated waveform could not be
checked.
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Other Complex Analog Properties
• From earlier preliminary discussion:
1. While ’a’ is true, the frequency of ’b’ is 100.0 Hz with
a 1.0% tolerance.
2. The mean slew rate of ’a’ from 0.0 V to 5.0 V is
250.0 V/s with a 1.0% tolerance.
3. The −3 dB low pass cut-off frequency from ’a’ to ’b’
is 25 Hz with a tolerance of 2.5%.
4. The gain-bandwidth product from ’a’ to ’b’ is 2.5
MHz with a tolerance of 1.0%.
5. Let T1 and T2 be two sets of crossing times of
various signals. Check that (maxT1
−minT1)+(maxT2 −minT2) < 2 max jitter .
•
(eye jitter)
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