Counting cycles on Cortex M0+

Question

I have a Cortex M0+ (SAML21) board that I'm using for performance testing. I'd like to measure how many cycles a given piece of code takes. I tried using DWT (DWT_CONTROL), but it never produced a result; it returned 0 cycles regardless of what code ran.

  // enable the use DWT
  *DEMCR = *DEMCR | 0x01000000;

  // Reset cycle counter
  *DWT_CYCCNT = 0;

  // enable cycle counter
  *DWT_CONTROL = *DWT_CONTROL | 1 ;
  // some code here
  // .....

  // number of cycles stored in count variable
  count = *DWT_CYCCNT;

Is there a way to count cycles (perhaps with an interrupt and counter?) much like I can query for milliseconds (eg. millis() on Arduino)?

this processor doesnt necessarily work that way, if the chip vendor (atmel now microchip) does not do any flash caching (like st) then you can time some code at a specific address with specific settings and get a count. but move that code to another alignment, change the clock settings, etc and the number of cpu clocks for that code can change a little or a lot. — old_timer
The cycle counter is an optional feature on the M0+. Are you sure this implementation actually has the feature? — Realtime Rik
@RealtimeRik, I am not sure. I'm dealing with the ATSAML21E18B, which dues have debug capabilities. But, I think andy mango is correct; I have to read those registers out via JTAG. — MarkP
Possibly. I am not sure without looking it up. I know on a M3/M4 you can read the cycle counter programatically as I have used it. — Realtime Rik

andy mango andy mango · Accepted Answer · 2018-01-28T01:52:42

According to the CMSIS header file for the M0+ (core_cm0plus.h), the Core Debug Registers are only accessible over the Debug Access Port and not via the processor. I can only suggest using some free running timer (maybe SysTick) or perhaps your debugger can be of some help to get access to the required registers.

Counting cycles on Cortex M0+

1 Answers