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Why do we use negedge of flop to get a TDO value in BSCAN?

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sathishkas

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Hi,

1) Could you please explain the reason why are we using negedge of flop to get a TDO value in the Boundary scan operation ? Why don't we get a value at posedge of clock?

2) Why Bypass Register is a 1-bit register? Without bypass register, why cant we bypass the operation?

Thanks,
Sathish
 

hello,

1) TDI is done at pos edge,now TDO is done at neg edge for just to avoid the hold violation,as through neg edge we provide the delay of half cycle.
2) Bypass register is just required for the board level testing where we need to do BS test another SoC on board.Tell me how can we do bypass without bypass register?
 

Hi,

1) For the first question, you said that to avoid a Hold violation we are taking the output value at negedge. Basically, these BSCAN registers form a shift register to get a TDO value & moreover, we are using only one clock called TCk then, where will we get a hold violation? Please explain this.

2) For Second question, if your bypass operation is selected as per your instruction, simply we can connect TDI pin to your TDO pin in such a case, why do we require a register in between TDI & TDO for Bypass operation?


Thanks,
Sathish
 

hello

1) For understanding,how we get the hold violation,you need to do some timing analysis on paper.Just do one thing,take any circuit with io pads,and do the jtag on paper with timing analysis,you will get the answer of your question.
2) How can you connect directly TDI to TDO pin? for that we required some logic in between TDI and TDO. BScan connect TDI to TDO pin directly through bypass register.
 

Hi,

2) If you have 15 chips connected in daisy chain mode and according to your solution TDI directly connected to TDO, How will you keep track of timing related information on board??
Consider 10 chips are in Bypass mode out of 15 chips. when will you expect the output at TDO of last chip?? After all you have to think in real silicon way.
Hence adding single register in between will add synchronous operation with respect to event signal TCK.
Hope you got the point.
 

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