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Bidirectional Bus Modelling
The document discusses various aspects of bus drivers and contention, including signal strength modeling and practical examples using Verilog syntax. It details different levels of strength (strong, weak, pull) and provides a specific instance of USB2 remote wakeup, describing how contention can occur and offering solutions to mitigate it. Overall, the focus is on managing bus contention through proper driver strength and logic signaling.
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Bidirectional Bus Modelling
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- 2. Agenda • Bus driversand Bus contention • Different signal strength • Modelling equivalence with actual analog model • Practical example of modelling
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- 5. • Basic fourkinds of drivers BUF I/O BUS STRONG1 I/P BUF I/O BUS PULL0 EN EN RpdEN RpuEN BUF BUF I/O BUS STRONG 0 I/P PULL1 I/P I/O BUS EN EN I/P Rpd RPU Vcc
- 6. • Contention Driver1 Driver2Bus value PULL1/PULL0 Strong1/Strong0 Strong1/Strong0 PULL1/PULL0 PULL0/PULL1 X PULL1/PULL0 PULL1/PULL0 PULL1/PULL0 Strong1/Strong0 Strong0/Strong1 X Strong1/Strong0 Strong1/Strong0 Strong1/Strong0
- 7. Different Signal Strength •Different Strength that can be modeled. STRENGTH LEVEL Supply 7 Strong 6 Pull 5 Large capacitance 4 Weak 3 Medium capacitance 2 Small capacitance 1 High Impedance 0
- 8. Analog Models • SupplyStrength: Diagram Syntax in Verilog BUF I/O BUS supply1 pwr_on ip(Supply) supply0 pwr_off EN
- 9. Analog Models • StrongStrength: Diagram Syntax in Verilog BUF I/O BUS strong1 sig_a i/p strong0 sig_b (Strong) EN
- 10. Analog Models • WeakStrength: Diagram Syntax in Verilog BUF I/O BUS weak1 sig_a i/p weak0 sig_b (weak) EN
- 11. Analog Models • PullStrength: Diagram Syntax in Verilog BUF i/o BUS Pull1 sig_a i/p Pull0 sig_b EN RpdEN Rpd
- 12. Signal Modeling Rules •Same strength but opposite logic value leads to bus contention in bus i.e. x • Higher strength always override the lower strength Examples: Pull0 Pull0 Pull0 X Weak1 Pull1
- 13. Example • USB2 RemoteWakeup What is USB2 Remote wakeup ?? -- If there are no transfers/bus is idle then usb2 device/host can go to sleep mode -- If device starts the host wakeup then that’s called Remote wakeup Protocol of Remote wakeup: -- Host drives Bus to idle Dp = 0 , Dm = 0. -- Device drive Dp = 0 and Dm = 1 for 15 ms -- Host will detect for 1 ms and drive Dp = 0 and Dm = 1 for 20 ms Chance of contention: -- If you see initially Dm = 0 and after that device needs to drive Dm = 1 -- If strength of Dm is same for host and device will same , it will lead to Bus contention on Dm line
- 14. Solution • Solution -- Hostwill drive Dm = 0 with strength weak0 -- Device will drive Dm = 1 with strength pull1 Syntax: Host – assign (weak0,weak1) dm = (if_remote_wakeup) ? If_dm : `bz; Device – assign (Pull0,Pull1) dm = (if_remote_wakeup) ? If_dm : `bz;
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