Electronics > Projects, Designs, and Technical Stuff
Distance between JTAG/SWD connector and MCU - what is too far?
ninux:
Dear colleagues,
Recently, one of our embedded gurus told me that I have placed the JTAG connector too far from the MCU to enable decent speeds (approx 40 mm, see attachment). However, it worked just fine with the settings our customer is using.
When I designed the board, I checked all reference guides for the MCU (STM32F412), but there was no info about trace lengths for the JTAG.
I know that JTAG can go up to pretty high speeds but do you consider these speeds for all of your designs having JTAG? What is the rule of thumb here? Should the JTAG connector be right next to the MCU in every design? What would you consider too far away?
Thank you for your replies =)
RoadRunner:
--- Quote from: ninux on July 28, 2020, 08:07:08 am ---Dear colleagues,
Recently, one of our embedded gurus told me that I have placed the JTAG connector too far from the MCU to enable decent speeds (approx 40 mm, see attachment). However, it worked just fine with the settings our customer is using.
When I designed the board, I checked all reference guides for the MCU (STM32F412), but there was no info about trace lengths for the JTAG.
I know that JTAG can go up to pretty high speeds but do you consider these speeds for all of your designs having JTAG? What is the rule of thumb here? Should the JTAG connector be right next to the MCU in every design? What would you consider too far away?
Thank you for your replies =)
--- End quote ---
JTAG is not that fast. 40 mm traces will not create issue for JTAG even in fastest mode. JTAG only goes few tens of Mhz.
It is common practice to put JTAG connector near to the target because of routing ease, definitely not because of speed.
If one put JTAG far away, Those JTAG traces need to routed on the board will criss cross with other traces, which only make layout a little bit hard.
Production programming/Test by a specific Tool/hardware/Test jig is only reason i can think of to put JTAG connector away from Target on specific place.
Regards
ataradov:
40 mm is nothing. The length does not matter that much. You want to be a bit more careful if you are using trace, but normal JTAG/SWD are quite tolerant to that stuff.
Siwastaja:
Your "embedded guru" sounds like a con.
ejeffrey:
The JTAG/swd cables I normally use are 15 cm. An few cm on the board is no big deal. I have run both significantly farther either to make the programming header more accessible or because I couldn't afford the board real estate closer to the target.
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