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brightblade
Joined: 11 Jan 2005
Posts: 18
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 28 Jun 2006 12:22 How to stay in BYPASS mode during JTAG Programming |
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Hi again....
Here comes my problem. I have FPGAs and PROMs on a pci card. I program them using JTAG. They use same TCK but TMS and TDI inputs are different. How can I give the BYPASS instruction to a device on the board. So that when the TCK comes the device stays unprogrammed. Anybody tell me the input sequence for TDI and TMS??? Thanks for your help...
(I think connecting TDI and TMS inputs to Vdd (logic1) makes the device stay unprogrammed although TCK clock comes. Am I right??)
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semiconductorman
Joined: 18 Dec 2004
Posts: 153
Helped: 19
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29 Jun 2006 14:49 Re: How to stay in BYPASS mode during JTAG Programming |
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If you hold TMS high for 5 TCK clocks continoulsy it will come to "TEST_RESET" state and it does not matter what you do with your TDI.
Regarding the BYPASS mode. If you look up your documentation it will tell you what needs to be scanned in to the instruction register to make the tap go to bypass mode .
Look up the statemachine explanation given in the ieee standard you'll understand else mail back with specific queries 
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brightblade
Joined: 11 Jan 2005
Posts: 18
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30 Jun 2006 8:41 Re: How to stay in BYPASS mode during JTAG Programming |
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BYPASS and reset are almost same to me at this point. Because in both of them my PROM stays unprogrammed. Thanks for your help. I also looked up the BYPASS mode instruction and I think it is complicated than just shifting ones from both inputs... Thanks for the help anyway
Added after 56 minutes:
Here is another question... it can be a silly one:D
There is a JTAG chain of xilinx PROMs on my PCB. I'm going to program these via JTAG. I will give the TCK TMS and TDI inputs properly. But how will I know the configuration is competed succesfully. Is there something for PROMs indicating successfull programming (maybe TDO any ideas) .
(for example in the same situation for FPGAs I look to the DONE pin if DONE is 1 then I understand that configuration successfull. But in PROM case there is no DONE pin.)
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semiconductorman
Joined: 18 Dec 2004
Posts: 153
Helped: 19
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30 Jun 2006 17:23 Re: How to stay in BYPASS mode during JTAG Programming |
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NO!  Jtag is just a 5 pin interface (trst, tdi, tms, tdo,tck) . You will not come to know anything ! What I would do if I where you was this. Scan-in the data you want to send twice (whole vector not bit by bit) . Let me try explaining by example, Suppose I want to scanin 8'b01011011 . First configure the instruction register to choose the right scan chain. then scanin 8'b01011011, now the value will be written to the scan register . If you wish to see if the data was properly written then you can repeat scanin 8'b01011011 . and see what comes out of TDO the pattern should be same8'b01011011 (the data that was first scanned in). But I don't know why you would do such a excercise everytime . May be at the most you could do it a couple of times to make sure that your scanin and scanout procedures are proper once this is established then don't do this excercise anymore. Hope this helps
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