Achieving Nanosecond Precise Timing using ASM

RE: Achieving Nanosecond Precise Timing using ASM

Vidar Berg — Mon, 27 Nov 2023 08:45:52 GMT

Sorry, I must have overlooked the fact that you wanted nanosecond precision when I wrote my reply. For nanosecond accuracy, you will need to create your own delay implementation as you initially tried. However, instead of using NOPs, you might get better results by basing your implementation on one of the delay functions in nrfx_coredep_delay_us.

[quote user="Michael Zeng"]Also, could you elaborate on what you mean by "requesting the 64M crystal oscillator (HFXO)"? Is the nRF52840 not already using its 64M crystal?[/quote]

It is running of the less accurate 64 MHz internal oscillator (HFINT) clock source by default.

RE: Achieving Nanosecond Precise Timing using ASM

Michael Zeng — Sat, 25 Nov 2023 02:46:10 GMT

Sorry, I'm not totally sure what you're suggesting. Form the first link you send, I agree that NOP does not produce a deterministic delay. However, the functions you linked all delay an integer number of microseconds, but I need another order of magnitude of precision--down to a hundred nanoseconds of 0.1 microseconds. Is there any way to achieve that without, for example modifying the functions in hal_nordic/nrfx/soc/nrfx_coredep.h at 56e0b052dff311c2f8eb08c6804e60fc79feb56f · zephyrproject-rtos/hal_nordic (github.com) (which I'm not sure would even work)?

Also, could you elaborate on what you mean by "requesting the 64M crystal oscillator (HFXO)"? Is the nRF52840 not already using its 64M crystal?

RE: Achieving Nanosecond Precise Timing using ASM

Vidar Berg — Thu, 23 Nov 2023 11:37:04 GMT

Hello,

The NOP instruction is not guaranteed to produce a deterministic delay: https://developer.arm.com/documentation/100235/0004/the-cortex-m33-instruction-set/miscellaneous-instructions/nop. Instead, I would recommend you try using k_busy_wait(). To further improve the timing, you may also consider requesting the 64M crystal oscillator (HFXO) as done in the radio test sample here: https://github.com/nrfconnect/sdk-nrf/blob/72e0f9a63481181994f35f4d2646117ed56d8778/samples/peripheral/radio_test/src/main.c#L11.

Delay function used by k_busy_wait(): https://github.com/zephyrproject-rtos/hal_nordic/blob/56e0b052dff311c2f8eb08c6804e60fc79feb56f/nrfx/soc/nrfx_coredep.h#L140

Best regards,

Vidar