This document discusses trends in mixed signal validation. It begins with an overview of mixed signal systems that contain both analog and digital components. The evolution of mixed signal validation is then described, from early approaches that simulated analog and digital components separately to modern tools that can jointly simulate both domains using languages like Verilog-AMS. The key steps in mixed signal validation are outlined, including modeling components in Verilog-AMS, validating blocks, and performing system-level validation. Throughout, the importance of accurate models for verification is emphasized. Examples of mixed signal modeling and a charge pump PLL validation environment are also provided.

1. Sunil Kumar
Intel, Bangalore
Trends in Mixed Signal Validation

2. Agenda
Mixed Signal System
Evolution of MSV
Verilog-AMS Overview
Mixed Signal Tools
Steps in MSV
Conclusion

3. Mixed Signal System
A system having some parts in analog domain and
some in digital domain
A typical chip has these mixed signal blocks:
PLLs and
CLK dist.
Power
Distribution
Thermal
Sensors
Resets
High Speed Interconnect
BIAS
Gen/Comp
Sideband IOs

4. Mixed Signal System: An Example
Serializer
De-
serializer
Clock and
Data
Recovery
PLL PLL
BIAS
Generation
BIAS
Generation

5. Evolution of MSV
Generation I
Analog Digital interactions were quite less
Analog was verified in CKT simulator while digital was
verified in digital simulator
A D were verified by manually review
Generation II
Analog blocks were modeled in Verilog/VHDL
These models were calibrated with CKT to some extent
Analog blocks were still verified in CKT simulators
A D were covered using the created models
The models were incapable of modeling analog behavior, e.g.,
voltage levels, ramp, termination effect etc

6. Evolution of MSV
Generation III
Share of Mixed signal circuits has grown rapidly
PLLs, DDR3/4, PCI Express, 10/40GbE, HyperTransport, FBD,
QPI
Digital models are not sufficient enough
HDLs have been created for analog and mixed signal
Verilog-AMS, VHDL-AMS
Analog and mixed signal blocks are modeled in AMS HDL
New tools came up to support these languages
Mentor’s ADMS, Cadence’s AMS, Synopsys’s Discovery-AMS
Most of these tools have started supporting Fast SPICE

7. SPICE
Verilog-A
Verilog-AMS Overview
Extension of Verilog HDL
Adds analog and mixed signal capabilities
Alignment of Verilog-AMS with the SystemVerilog is WIP
Verilog-D
MS Extension
Verilog-AMS

8. Verilog-AMS: Examples
`include “disciplines.vams”
module res10ohm (
res_in,
res_out
);
inout res_in, res_out;
electrical res_in, res_out;
parameter real RES_VALUE = 10.0;
analog begin
V(res_in, res_out) <+
RES_VALUE*I(res_in, res_out);
end
endmodule
`include “disciplines.vams”
module dig2ana (
dig_in,
ana_out
);
input dig_in;
output ana_out;
wire dig_in;
electrical ana_out;
real ana_value;
analog begin
if (dig_in == 1) ana_value = 1.1;
else ana_value = 0.0;
V(ana_out) <+ transition(ana_value, 0,
10e-12);/
end
endmodule

9. On Modeling
Validation is as good as the models
The models need to be accurate enough
And for speed, they need to have enough abstractions
Verilog-AMS
Verilog/SV SPICE
AccuracySpeed

10. Mixed Signal Tools
Combine two different methods of simulation
Event-driven simulation as found in logic simulators
Continuous-time simulation as found in circuit simulators
Use digital simulator and analog simulator
Mentor’s ADMS : Modelsim + Eldo
Synopsys’s Discovery-AMS: VCS + Nanosim
Cadence’s AMS: NC-Sim + Spectre
On top of these the tool has to
Split the netlist in Verilog and SPICE and insertion of connect
modules
Set up and run the simulation by synchronizing the time steps of
two simulators

11. Steps in MSV
Step 1: Modeling
Identify the CKT blocks which needs to be modeled in AMS
Create AMS models with inputs from CKT team
Extensively calibrate these models with the corresponding CKT blocks
Step 2: Block Validation
Extensive validation of all the closed loops and complex features in lower level
testbenches
Each analog block can have any one of the views: Digital, AMS, SPICE
Step 3: Cluster/System Level Validation
Validate the sequence and inter-dependencies of the features validated at block
level
Most of the traditional digital validation techniques can be used, e.g.,
randomization, coverage, Specman/SV based testbenches etc.
Reuse of corresponding digital validation environment is possible

12. Example: Charge Pump PLL
Ref: August, N., “A Robust and Efficient Pre-Silicon Validation Environment for Mixed-Signal Circuits on Intel's Test Chips”,
ISQED 2008

13. Analog Driver (Verilog-AMS Module)
Test (SV Program)
Harness (SV Class)
MSV Testbench in SV
BFM1 (SV Class)
BFMN (SV Class)
Checker1 (SV Class)
CheckerN (SV Class)
Interface1 InterfaceN
DUT (Mixed Analog and Digital)
Clock
Generator
Analog
Sources
Reset
Generator
SV Wrapper (SV Module)
Top Level (SV Module)
Ref: August, N., “A Robust and Efficient Pre-Silicon Validation Environment for Mixed-Signal Circuits on Intel's Test Chips”, ISQED 2008
Analog
Digital

14. Conclusions
Mixed Signal Validation has become an important component of
the validation space
The MSV tools have matured
Many fatal bugs which were earlier found only in post-silicon
can be uncovered in MSV
Most of the digital validation techniques can be used

15. References
Verilog-AMS Language Reference Manual
http://www.verilog.org/verilog-ams/htmlpages/public-
docs/lrm/2.3/VAMS-LRM-2-3.pdf
“The Designer’s Guide to Verilog-AMS”, Kenneth S.
Kundert and Olaf Zinke, Springer, 2004
Verilog-AMS models examples:
http://www.designers-guide.org/VerilogAMS/

16. Q&A