Minimum clock speed for SWD?

RE: Minimum clock speed for SWD?

Roger Clark — Sun, 13 Aug 2017 11:01:24 GMT

@curtisHx

What processor are you using to bit-bang the SWD.

Only managing 10khz is quite slow for modern processors.

I know bit-banged I2C on a 72Mhz STM32F103 can be made to run at 400kHz without too many optimisations.

Are you running on an old slow processor ??

RE: Minimum clock speed for SWD?

slowcoder — Sun, 13 Aug 2017 07:48:07 GMT

I guess that depends on what state SWCLK/SWDIO is in when it stops, and for how long a "moment" is. If both lines are low for 100uS or longer (not entirely sure about this value), I guess the nRF51 could interpret it as a reset due to the shared SWDIO/nRESET functionality.

However, if you're just talking about very minor changes in timing, it seems to handle that well. Every now and then the MCU that I'm implementing this on wants to service an IRQ or something else, and I get a SWCLK that's about twice as long. The nRF51 seems to handle this just fine.

RE: Minimum clock speed for SWD?

slowcoder — Sat, 12 Aug 2017 10:12:39 GMT

In case anyone else plays around with very slow SWD (and, as this is the only relevant discussion on the internet about the slowest SWD speed), I just discovered that the nRF51 does indeed not ACK (or, clock out anything) if you're running at 18KHz. After clocking up to 190KHz (no other changes), the nRF51 started to respond.

RE: Minimum clock speed for SWD?

CurtisHx — Fri, 23 Dec 2016 14:16:46 GMT

Thanks for checking that out. Turns out the code wasn't correctly calculating the parity bit, and the nRF52 wasn't acking per the SWD spec. So it looks like we can use our 10khz clock, just with the understanding that we're outside of the spec and things may not work correctly.

RE: Minimum clock speed for SWD?

Susheel Nuguru — Fri, 23 Dec 2016 07:51:52 GMT

I see that this information seems to be missing in the nRF52 manuals. I would assume that it should be the same requirement as for nRF51 which was a minimum of 125 KHz mentioned here. 10 KHz that you have on your system seems too less.