1. Switch-Level Modeling How to describe a switch-level circuit ?

2. Verilog Switch Primitives Modeling transistor networks at the switch-level more accurately represents their operation. Verilog provides unidirectional and bidirectional primitives that you can use to model the switch networks: The following are unidirectional primitives: cmos nmos pmos pullup rcmos rnmos rpmos pulldown The following are bidirectional primitives: tran tranif0 tranif1 rtran rtranif0 rtranif1

3. Switch Instantiation Source Gate Drain nmos(drain,source,gate);

4. Switch Instantiation Source Gate Drain pmos(drain,source,gate);

5. Switch Instantiation Source Gate Drain nmos(drain,source,gate);

6. Switch Instantiation Source Drain pgate ngate cmos (drain, source, ngate, pgate)

7. Switch Instantiation data1 data2 control tranif0 (data1, data2, control); tranif1 (data1, data2, control);

8. Switch Delay You can assign delays to some switch types:  The unidirectional coms,nmos,and pmos switches can have rise,fall,and turn-off delays coms #( ) (d, s, ng, pg); nmos #(, ) (d, s, g); pmos #(, ), ) (d, s, g);  The bidirectional switches tranif0 and tranif1 can have turn-on and turn-off delays,but no source-drain channel delays tranif0 #( ) (d, s, g); tranif1 #(, ) (d, s, g);  The pulldown,pullup,and tran gates cannot have delays Note: You can specify delays in min:typ:max format.

9. Drive Strength You can assign strengths to some primitive types:  The pulldown and pullup primitives can have one or two drive strengths the simulator ignores the unneeded strength specification pullup (weak1, weak0) (net1);  The boolean primitives can have two drive strengths You must specify both drive strengths,or none nand (highz1, strong) (net1,net2,net3);  The trireg net type can have charge strengths trireg (small) net1;  The switch primitives CANNOT have drive strengths! Level 7 6 5 4 3 2 1 0 Drive supply strong pull weak highz Charge large medium small

10. Strength Reduction The switches can reduce the strength of signals passing through them:  The cmos,nmos,pmos,tran,tranif0,and tranif1 primitives reduce a supply strength signal to a strong signal  The rcmos,rnmos,rpmos,rtran,rtranif0,and rtranif1 primitives reduce signal strength according to the following table: Input strength Reducel strengh 7 – supply 5 – pull 6 – strong 5 – pull 5 – pull 3 – weak 4 – large 2 – medium 3 – weak 2 – medium 2 – medium 1 – small 1 – small 1 – small 0 – highz 0 – highz

11. Switch-Level Networks Switch networks may contain unidirectional and bidirectional switches. Verilog-XL partitions switch-level networks into channel-connected regions.

12. The Switch-XL Algorithm Use the Switch-XL algorithm to:  Accelerate simulation of bidirectional switches --- The XL algorithm does not accelerate bidirectional switches  Simulate up to 250 relative drive strengths on switches  Simulate up to 250 relative charge strengths on trireg nets s3s3 s2s2 s1s1 Transistor Strengths 2 1 1

13. Selecting the Switch-XL Algorithm Use the +switchxl option to globally enable the Switch-XL algorithm. verilog source.v +switchxl Use the `switch compiler directive to selectively enable the Switch-XL algorithm. `switch XL // control networks here `switch default // datapath networks here Use the Switch-XL algorithm: ---For a significantly –sized network of bidirectional switches ---For a network of switches you cannot otherwise correctly functionally model with only 2 switch drive strengths and 4 (including none)net charge strengths Use the default algorithm: ---For a network of densely-packed significantly-sized regularly-structured pass transistors

14. The Switch-XL Strength Model The drive strength expression must evaluate to a number from 1 to 250. These statements declare tran switches and assign relative drive strengths. Switch t1 has the largest conductance relative to t0 and rt. tranif1 strength(3) t1 (s0,d0,g0); tranif0 strength(2) t0 (s1,d1,g1); rtran strength(1) rt (s2,d2); The charge strength expression must evaluate to a number from 0 to 250. These statements declare trireg nets and assign relative charge strengths. Net a has the largest capacitance relative to b,c and d. trireg strength(25) a; trireg strength(10) b; trireg strength(5) c; trireg strength(1) d;

15. Switch-XL Strength Reduction The Switch-XL and default algorithms reduce signal strength differently:  The default algorithm: --- Reduces signal strength by 0,1,or 2 levels for each switch instance --- An rtran reduces a weak drive to a medium charge  The Switch-XL algorithm: --- Treats all drive strengths as higher than charge strengths --- Maps standard strengths into the range of network strengths --- Reduces strength once (by the highest resistance) in the channel pulluppulldown Pu1 StX We1 StX Me1 StX Me0 StX We0 StX Pu0 StX Default Switch-Xl pulluppulldown Pu1 St1 We1 St1 Me1 St1 Me0 St0 We0 St0 Pu0 St0 Default Switch-Xl (same channel)

16. How Switch-XL Works The Switch-XL algorithm performs the following steps: 1. Detects channel-connected switch networks containing at least one bidirectional switch 2. Converts the timing model of unidirectional switches in these networks from the rise/fall/turn-off model to the turn-on/turn-off model of bidirectional switches 3. Optimizes these networks,including removing nets 4. Compiles these networks into accelerative expressions for the XL engine vdd a b y vss Switch-XL can remove net n1 n1

17. Timing Model Conversion In channel-connected regions containing at least one bidirectional switch, Switch-XL converts the unidirectional switch timing model to the bidirectional switch timing model,so that it can accelerate the region:  It converts rise/fall delays into turn-on/turn-off delays --- The rise delay becomes the turn-on delays,fall becomes turn-off  It converts rise/fall/turn-off delays into turn-on/turn-off delays --- The smaller of the rise and fall delays becomes the turn-on delay nmos tranif1 not convertedtiming model converted

18. Summary In this section, you learned about:  Switch-level modeling  The Switch-XL algorithm  Choose an acceleration algorithm

19. Review 1. Name the Verilog bidirectional primitives. 2. How do the delay models for the unidirectional and bidirectional switches differ? 3. A Verilog-XL switch-level network contains which types of switches? 4. What is the range of charge strengths you can assign to a Switch-XL trireg net? 5. How does Switch-XL reduce signal strength in channel?