RE: FreeRTOS time inaccuracy. Drift when compared to RTC

Susheel Nuguru — Tue, 25 Jan 2022 09:13:19 GMT

Hi,

The problem lies here

#define configTICK_RATE_HZ 1000

This tick rate is impossible to realize with the RTC which is working on a 32768 Hz clock. There is no correct prescalar in the RTC registers which you can set to get exactly 1000 ticks per seconds. You should set this to 1024 as below so that

#define configTICK_RATE_HZ 1024

And you cannot get exactly 1ms tick with RTC, you get 0.976ms instead.

If you want better accuracy then you need to use system clock which is more accurate but uses higher power consumption.

RE: FreeRTOS time inaccuracy. Drift when compared to RTC

xgroleau — Mon, 24 Jan 2022 16:22:28 GMT

Hi Jonathan,

I've updated the timer callback a bit to get more information.

The RTC1 is the one used by FreeRTOS and the RTC2 is the one I've created manually. The RTC2 is on time even after an hour of runtime, while the RTC1, and thus FreeRTOS, is lagging a little bit behind.

extern uint64_t gs_time_counter;
void test_timer(){
    LOG_INFO("RTOS: %d, RTC1: %d, RTC2: %d", xTaskGetTickCount(), NRF_RTC1->COUNTER, NRF_RTC2->COUNTER);
}
void timers_init(void)
{
      TimerHandle_t m_timer = xTimerCreate("test",
                                     HEART_RATE_MEAS_INTERVAL,
                                     pdTRUE,
                                     NULL,
                                     test_timer);
    xTimerStart(m_timer, OSTIMER_WAIT_FOR_QUEUE);
    // Rest of code
    
}

Here is the output

<info> app: RTOS: 11000, RTC1: 11000, RTC2: 110
<info> app: RTOS: 12000, RTC1: 12000, RTC2: 120
<info> app: RTOS: 13000, RTC1: 13000, RTC2: 130
<info> app: RTOS: 14000, RTC1: 14000, RTC2: 141
<info> app: RTOS: 15000, RTC1: 15000, RTC2: 151
<info> app: RTOS: 16000, RTC1: 16000, RTC2: 161
<info> app: RTOS: 17000, RTC1: 17000, RTC2: 171
<info> app: RTOS: 18000, RTC1: 18000, RTC2: 181
<info> app: RTOS: 19000, RTC1: 19000, RTC2: 191
<info> app: RTOS: 20000, RTC1: 20000, RTC2: 201
<info> app: RTOS: 21000, RTC1: 21000, RTC2: 211
<info> app: RTOS: 22000, RTC1: 22000, RTC2: 221
<info> app: RTOS: 23000, RTC1: 23000, RTC2: 231
<info> app: RTOS: 24000, RTC1: 24000, RTC2: 241
<info> app: RTOS: 25001, RTC1: 25001, RTC2: 251
<info> app: RTOS: 26000, RTC1: 26000, RTC2: 261
<info> app: RTOS: 27000, RTC1: 27000, RTC2: 271
<info> app: RTOS: 28000, RTC1: 28000, RTC2: 281
<info> app: RTOS: 29000, RTC1: 29000, RTC2: 292
<info> app: RTOS: 30000, RTC1: 30000, RTC2: 302
<info> app: RTOS: 31000, RTC1: 31000, RTC2: 312
<info> app: RTOS: 32000, RTC1: 32000, RTC2: 322
<info> app: RTOS: 33000, RTC1: 33000, RTC2: 332
<info> app: RTOS: 34000, RTC1: 34000, RTC2: 342
<info> app: RTOS: 35000, RTC1: 35000, RTC2: 352
<info> app: RTOS: 36000, RTC1: 36000, RTC2: 362
<info> app: RTOS: 37000, RTC1: 37000, RTC2: 372
<info> app: RTOS: 38000, RTC1: 38000, RTC2: 382
<info> app: RTOS: 39000, RTC1: 39000, RTC2: 392
<info> app: RTOS: 40001, RTC1: 40001, RTC2: 402
<info> app: RTOS: 41001, RTC1: 41001, RTC2: 412
<info> app: RTOS: 42000, RTC1: 42000, RTC2: 422
<info> app: RTOS: 43001, RTC1: 43001, RTC2: 433
<info> app: RTOS: 44000, RTC1: 44000, RTC2: 443
<info> app: RTOS: 45000, RTC1: 45000, RTC2: 453
<info> app: RTOS: 46000, RTC1: 46000, RTC2: 463
<info> app: RTOS: 47000, RTC1: 47000, RTC2: 473
<info> app: RTOS: 48000, RTC1: 48000, RTC2: 483
<info> app: RTOS: 49000, RTC1: 49000, RTC2: 493
<info> app: RTOS: 50000, RTC1: 50000, RTC2: 503
<info> app: RTOS: 51000, RTC1: 51000, RTC2: 513
<info> app: RTOS: 52000, RTC1: 52000, RTC2: 523
<info> app: RTOS: 53000, RTC1: 53000, RTC2: 533
<info> app: RTOS: 54000, RTC1: 54000, RTC2: 543
<info> app: RTOS: 55001, RTC1: 55001, RTC2: 553
<info> app: RTOS: 56000, RTC1: 56000, RTC2: 563
<info> app: RTOS: 57000, RTC1: 57000, RTC2: 574
<info> app: RTOS: 58000, RTC1: 58000, RTC2: 584
<info> app: RTOS: 59000, RTC1: 59000, RTC2: 594
<info> app: RTOS: 60000, RTC1: 60000, RTC2: 604
<info> app: RTOS: 61000, RTC1: 61000, RTC2: 614
<info> app: RTOS: 62000, RTC1: 62000, RTC2: 624
<info> app: RTOS: 63000, RTC1: 63000, RTC2: 634
<info> app: RTOS: 64000, RTC1: 64000, RTC2: 644
<info> app: RTOS: 65000, RTC1: 65000, RTC2: 654
<info> app: RTOS: 66000, RTC1: 66000, RTC2: 664
<info> app: RTOS: 67000, RTC1: 67000, RTC2: 674
<info> app: RTOS: 68000, RTC1: 68000, RTC2: 684
<info> app: RTOS: 69000, RTC1: 69000, RTC2: 694
<info> app: RTOS: 70000, RTC1: 70000, RTC2: 704
<info> app: RTOS: 71000, RTC1: 71000, RTC2: 715
<info> app: RTOS: 72000, RTC1: 72000, RTC2: 725
<info> app: RTOS: 73000, RTC1: 73000, RTC2: 735
<info> app: RTOS: 74000, RTC1: 74000, RTC2: 745
<info> app: RTOS: 75000, RTC1: 75000, RTC2: 755
<info> app: RTOS: 76000, RTC1: 76000, RTC2: 765
<info> app: RTOS: 77000, RTC1: 77000, RTC2: 775
<info> app: RTOS: 78000, RTC1: 78000, RTC2: 785
<info> app: RTOS: 79000, RTC1: 79000, RTC2: 795
<info> app: RTOS: 80000, RTC1: 80000, RTC2: 805
<info> app: RTOS: 81000, RTC1: 81000, RTC2: 815
<info> app: RTOS: 82000, RTC1: 82000, RTC2: 825
<info> app: RTOS: 83000, RTC1: 83000, RTC2: 835
<info> app: RTOS: 84000, RTC1: 84000, RTC2: 845
<info> app: RTOS: 85000, RTC1: 85000, RTC2: 855
<info> app: RTOS: 86000, RTC1: 86000, RTC2: 866
<info> app: RTOS: 87000, RTC1: 87000, RTC2: 876
<info> app: RTOS: 88000, RTC1: 88000, RTC2: 886
<info> app: RTOS: 89000, RTC1: 89000, RTC2: 896
<info> app: RTOS: 90000, RTC1: 90000, RTC2: 906
<info> app: RTOS: 91000, RTC1: 91000, RTC2: 916
<info> app: RTOS: 92000, RTC1: 92000, RTC2: 926
<info> app: RTOS: 93000, RTC1: 93000, RTC2: 936
<info> app: RTOS: 94000, RTC1: 94000, RTC2: 946
<info> app: RTOS: 95000, RTC1: 95000, RTC2: 956
<info> app: RTOS: 96000, RTC1: 96000, RTC2: 966
<info> app: RTOS: 97001, RTC1: 97001, RTC2: 976
<info> app: RTOS: 98000, RTC1: 98000, RTC2: 986
<info> app: RTOS: 99000, RTC1: 99000, RTC2: 996
<info> app: RTOS: 100000, RTC1: 100000, RTC2: 1007
<info> app: RTOS: 101000, RTC1: 101000, RTC2: 1017
<info> app: RTOS: 102000, RTC1: 102000, RTC2: 1027
<info> app: RTOS: 103000, RTC1: 103000, RTC2: 1037
<info> app: RTOS: 104000, RTC1: 104000, RTC2: 1047
<info> app: RTOS: 105000, RTC1: 105000, RTC2: 1057
<info> app: RTOS: 106000, RTC1: 106000, RTC2: 1067
<info> app: RTOS: 107000, RTC1: 107000, RTC2: 1077
<info> app: RTOS: 108000, RTC1: 108000, RTC2: 1087
<info> app: RTOS: 109000, RTC1: 109000, RTC2: 1097
<info> app: RTOS: 110000, RTC1: 110000, RTC2: 1107
<info> app: RTOS: 111000, RTC1: 111000, RTC2: 1117
<info> app: RTOS: 112000, RTC1: 112000, RTC2: 1127
<info> app: RTOS: 113000, RTC1: 113000, RTC2: 1137
<info> app: RTOS: 114000, RTC1: 114000, RTC2: 1148
<info> app: RTOS: 115001, RTC1: 115001, RTC2: 1158
<info> app: RTOS: 116000, RTC1: 116000, RTC2: 1168
<info> app: RTOS: 117000, RTC1: 117000, RTC2: 1178
<info> app: RTOS: 118000, RTC1: 118000, RTC2: 1188
<info> app: RTOS: 119000, RTC1: 119000, RTC2: 1198
<info> app: RTOS: 120000, RTC1: 120000, RTC2: 1208
<info> app: RTOS: 121001, RTC1: 121001, RTC2: 1218
<info> app: RTOS: 122001, RTC1: 122001, RTC2: 1228
<info> app: RTOS: 123000, RTC1: 123000, RTC2: 1238
<info> app: RTOS: 124000, RTC1: 124000, RTC2: 1248
<info> app: RTOS: 125000, RTC1: 125000, RTC2: 1258
<info> app: RTOS: 126000, RTC1: 126000, RTC2: 1268
<info> app: RTOS: 127000, RTC1: 127000, RTC2: 1278
<info> app: RTOS: 128000, RTC1: 128000, RTC2: 1289
<info> app: RTOS: 129002, RTC1: 129002, RTC2: 1299
<info> app: RTOS: 130000, RTC1: 130000, RTC2: 1309
<info> app: RTOS: 131000, RTC1: 131000, RTC2: 1319
<info> app: RTOS: 132000, RTC1: 132000, RTC2: 1329
<info> app: RTOS: 133000, RTC1: 133000, RTC2: 1339
<info> app: RTOS: 134000, RTC1: 134000, RTC2: 1349
<info> app: RTOS: 135000, RTC1: 135000, RTC2: 1359
<info> app: RTOS: 136000, RTC1: 136000, RTC2: 1369
<info> app: RTOS: 137001, RTC1: 137001, RTC2: 1379
<info> app: RTOS: 138000, RTC1: 138000, RTC2: 1389
<info> app: RTOS: 139000, RTC1: 139000, RTC2: 1399
<info> app: RTOS: 140000, RTC1: 140000, RTC2: 1409
<info> app: RTOS: 141000, RTC1: 141000, RTC2: 1419
<info> app: RTOS: 142000, RTC1: 142000, RTC2: 1429
<info> app: RTOS: 143000, RTC1: 143000, RTC2: 1440
<info> app: RTOS: 144000, RTC1: 144000, RTC2: 1450
<info> app: RTOS: 145000, RTC1: 145000, RTC2: 1460
<info> app: RTOS: 146000, RTC1: 146000, RTC2: 1470
<info> app: RTOS: 147000, RTC1: 147000, RTC2: 1480
<info> app: RTOS: 148000, RTC1: 148000, RTC2: 1490
<info> app: RTOS: 149000, RTC1: 149000, RTC2: 1500
<info> app: RTOS: 150000, RTC1: 150000, RTC2: 1510
<info> app: RTOS: 151000, RTC1: 151000, RTC2: 1520

We can see that the RTC1 and FreeRTOS have the same tick, which means it's the RTC1 is wrongly configured.
The delay adds up and when the RTC2 is at second 152, the RTC1 is at 151.

I hope this information is useful.

Regards,
Xavier