SDK • DMA Time Has Large Variation

Code:

[Time dma transfers of 40 bytes over 100 loopsUse systick_hw->cvrFirst time, Sys ticks 162Start tick number 16777095, end 16776932, delta is 163Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16775465, delta is 1747Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74Start tick number 16777212, end 16777138, delta is 74After 100 tries, min = 74, max = 1747, mean = 91 ticks/code]And, the source:[code]/** * Copyright (c) 2020 Raspberry Pi (Trading) Ltd. * * SPDX-License-Identifier: BSD-3-Clause * * 20250528 tomdean - modified to use systick_hw->cvr top time the dma transaction * */// Use the DMA to copy data between two buffers in memory.#include <stdio.h>#include "pico/stdlib.h"#include "hardware/dma.h"#include "hardware/structs/systick.h" // access the systick register which runs at sysclk#define BUF_SIZE 40 #define NUM_LOOPS 100// Data will be copied from src to dstuint8_t src[BUF_SIZE] ;uint8_t dst[BUF_SIZE] ;int main() {    uint32_t start, end, delta, min, max, sum;    int idx ;    stdio_init_all() ;    systick_hw->csr = 0x5 ; // clksource processor clock, enable counter    systick_hw->rvr = 0x00FFFFFF ;    sleep_ms(1000) ; // give the usb system time to setup    printf("\nTime dma transfers of %d bytes over %d loops\n", BUF_SIZE, NUM_LOOPS) ;    // Get a free channel, panic() if there are none    int chan = dma_claim_unused_channel(true) ;    // 8 bit transfers. Both read and write address increment after each    // transfer (each pointing to a location in src or dst respectively).    // No DREQ is selected, so the DMA transfers as fast as it can.    dma_channel_config c = dma_channel_get_default_config(chan) ;    channel_config_set_transfer_data_size(&c, DMA_SIZE_8) ;    channel_config_set_read_increment(&c, true) ;    channel_config_set_write_increment(&c, true) ;    dma_channel_configure(        chan,          // Channel to be configured        &c,            // The configuration we just created        dst,           // The initial write address        src,           // The initial read address        BUF_SIZE,      // Number of transfers ;        false          // Do not start immediately.    ) ;    // initialize the source buffer    for (idx=0; idx<BUF_SIZE; idx++) {        src[idx] = 0x10 + idx ;    }    printf("Use systick_hw->cvr\n");    start = systick_hw->cvr ; // cvr counts down    dma_start_channel_mask(1u << chan) ;    // We could choose to go and do something else whilst the DMA is doing its    // thing. In this case the processor has nothing else to do, so we just    // wait for the DMA to finish.    dma_channel_wait_for_finish_blocking(chan) ;    // The DMA has now copied our text from the transmit buffer (src) to the    // receive buffer (dst), so we can print it out from there.    printf("First time, Sys ticks %lu\n", start - systick_hw->cvr) ; // cvr counts down    min = 99999999;    max = 0;    sum = 0;    for (idx=0; idx<NUM_LOOPS; idx++) {        systick_hw->cvr = 0 ; // any write clears cvr and disables counter        systick_hw->csr = 0x5 ; // clksource processor clock, enable counter        start = systick_hw->cvr ; // cvr counts down        dma_channel_set_read_addr(chan,src, false) ; // don't start now        dma_channel_set_write_addr(chan, dst, true) ; // start now        dma_channel_wait_for_finish_blocking(chan) ;        end = systick_hw->cvr ; // cvr counts down        delta = start - end ; // cvr counts down        if (delta < min) min = delta ;        if (delta > max) max = delta ;        sum += delta ;        printf("Start tick number %lu, end %lu, delta is %lu\n", start, end, delta) ;    }    //printf("\n") ;    printf("After %d tries, min = %lu, max = %4lu, mean = %lu ticks\n",           NUM_LOOPS, min, max, sum / NUM_LOOPS) ;#if 0    printf("Use time_us_32()\n");    min = 99999999;    max = 0;    sum = 0;    for (idx=0; idx<NUM_LOOPS; idx++) {        start = time_us_32() ;        dma_channel_set_read_addr(chan,src, false) ; // don't start now        dma_channel_set_write_addr(chan, dst, true) ; // start now        dma_channel_wait_for_finish_blocking(chan) ;        delta = time_us_32() - start ;        if (delta < min) min = delta ;        if (delta > max) max = delta ;        sum += delta ;        //printf(" %lu", delta) ;    }    //printf("\n") ;    printf("After %d tries, min = %lu, max = %lu, mean = %lu usec\n",           NUM_LOOPS, min, max, sum / NUM_LOOPS) ;#endif    while (1) ; // don't run off the end of the world!}