Electronics > Projects, Designs, and Technical Stuff

Distance between JTAG/SWD connector and MCU - what is too far?

<< < (3/3)

ataradov:
It is probably fine. This is 100 MHz device, so trace is probably works at max 50 MHz. It is not that fast.

And if you want to put series resistors, put them on all trace pins, not just clock. In reality it does not matter.

I would be more worried about those resistors to ground. I would remove those. They are not needed and have a chance to create issues.

Vestom:

--- Quote from: ataradov on July 29, 2020, 05:45:47 am ---It is probably fine. This is 100 MHz device, so trace is probably works at max 50 MHz. It is not that fast.

--- End quote ---
When discussing signal integrity, it is not the frequency of the signal that matters, but the rise/fall-times of the edges. I have seen SI issues on 16 MHz clock signals with fast edges.


--- Quote from: ataradov on July 29, 2020, 05:45:47 am ---And if you want to put series resistors, put them on all trace pins, not just clock. In reality it does not matter.

--- End quote ---
TMS, TDO and TDI are level sensitive signals, so they are not as critical as TCK, which is edge sensitive. If board space is tight, a resistor on TCK should be prioritized.
[/quote]


--- Quote from: ataradov on July 29, 2020, 05:45:47 am ---I would be more worried about those resistors to ground. I would remove those. They are not needed and have a chance to create issues.

--- End quote ---
I do not agree. It is good practice to tie all inputs, unless the device is known to have reliable internal pull-up/down. What kind of issues do you foresee?

When giving advice, it is good practice not only to tell "what", but also "why", to make the receiver able to gain understanding and to evaluate the quality of the advice.

ataradov:
Trace port uses DDR signaling, so both rising and falling edge of the clock matters. Introducing any asymmetry has potential to cause issues. From the MCU point  of view trace pins are always outputs, so pull-down resistors don't really do anything when trace is active. And if debugger creators feel like their debugger inputs need a stable level, they will include resistors on their end.

ninux:

--- Quote from: ataradov on July 29, 2020, 06:23:39 am ---Trace port uses DDR signaling, so both rising and falling edge of the clock matters. Introducing any asymmetry has potential to cause issues. From the MCU point  of view trace pins are always outputs, so pull-down resistors don't really do anything when trace is active. And if debugger creators feel like their debugger inputs need a stable level, they will include resistors on their end.

--- End quote ---

To satisfy all possible opinions on this, one could prepare pull-down resistors as I did and the customer can decide if they want them populated or not I guess  ;D

Vestom:

--- Quote from: ataradov on July 29, 2020, 06:23:39 am ---Trace port uses DDR signaling, so both rising and falling edge of the clock matters. Introducing any asymmetry has potential to cause issues. From the MCU point  of view trace pins are always outputs, so pull-down resistors don't really do anything when trace is active. And if debugger creators feel like their debugger inputs need a stable level, they will include resistors on their end.

--- End quote ---
I agree, there is no need to put pull-down resistors on outputs ;D (Though 10k is very unlikely to cause any asymmetry...)

Personally, I would rather have put series resistors on these signals to be able to trim impedance and provide a minimum of protection on a connector interface. But that was not the question ;)

Navigation

[0] Message Index

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod