nRF52840 SDK16 - RTC Driver general questions - instance, initialization, and channels

Question

Hi everyone, 
 I want to use the RTC module in my code, and I have some questions. 
 1. What I have understood so far is that the nRF52840 has three RTCs (RTC0, RTC1, RTC2). RTC0 is used by the SD and RTC1 from the application timer software. So it's safer using RTC2 for events generation, right? 
 2. What is the id parameter when calling the NRFX_RTC_INSTANCE macro for creating an RTC instance? For example, if I use RTC2 the id value must be 2, while if I use RTC2 the id value must be 1 ? 
 3. The PRESCALER register is a 12-bit register, meaning that the maximum value is 4095, giving a tick every 125 ms. So, with the TICK event, the maximum event period is limited to 125ms? 
 4. Each RTC has four compare channels, right? The function to set a compare channel is nrfx_rtc_cc_set . According to the function's description, the second parameter is the channel of the instance. So the value of this parameter could be 1,2,3, or 4? For example, can I use all of the four compare channels of RTC2 like this? 
 static void rtc_config(void) {
 uint32_t err_code;

 // Initialize RTC instance
 nrf_drv_rtc_config_t config = NRF_DRV_RTC_DEFAULT_CONFIG;
 config.prescaler = 327; //100Hz (10ms)
 err_code = nrf_drv_rtc_init(&rtc, &config, rtc_handler);
 APP_ERROR_CHECK(err_code);

 err_code = nrf_drv_rtc_cc_set(&rtc, 0, 1, true); // compare event occurs every 10ms at channel 0
 APP_ERROR_CHECK(err_code);
 
 err_code = nrf_drv_rtc_cc_set(&rtc, 1, 2, true); // compare event occurs every 20ms at channel 1
 APP_ERROR_CHECK(err_code);
 
 err_code = nrf_drv_rtc_cc_set(&rtc, 2, 3, true); // compare event occurs every 30ms at channel 2
 APP_ERROR_CHECK(err_code);
 
 err_code = nrf_drv_rtc_cc_set(&rtc, 3, 4, true); // compare event occurs every 40ms at channel 3
 APP_ERROR_CHECK(err_code);

 // Power on RTC instance
 nrf_drv_rtc_enable(&rtc);
} 
 
 Thanks 
 Nick

ovrebekk · Accepted Answer

Hi Nick 
 1. What I have understood so far is that the nRF52840 has three RTCs (RTC0, RTC1, RTC2). RTC0 is used by the SD and RTC1 from the application timer software. So it's safer using RTC2 for events generation, right? 
 This is correct. The SoftDevice uses RTC0, and the app_timer uses RTC1, so if you use both then only RTC2 will be available to the application. 
 Are you sure that you could not just use app_timer callbacks for your event generation? 
 2. What is the id parameter when calling the NRFX_RTC_INSTANCE macro for creating an RTC instance? For example, if I use RTC2 the id value must be 2, while if I use RTC2 the id value must be 1 ? 
 Yes. The ID parameter simply refers to the peripheral instance ID, so id 0 means RTC0, id 1 means RTC1 and so on. 
 If you pick an ID that is already in use you should get an error message when compiling. 
 3. The PRESCALER register is a 12-bit register, meaning that the maximum value is 4095, giving a tick every 125 ms. So, with the TICK event, the maximum event period is limited to 125ms? 
 That is correct. If you want a longer event period you need to use a compare register and enable one of the compare events. 
 4. Each RTC has four compare channels, right? The function to set a compare channel is nrfx_rtc_cc_set . According to the function's description, the second parameter is the channel of the instance. So the value of this parameter could be 1,2,3, or 4? For example, can I use all of the four compare channels of RTC2 like this? 
 This is detailed here . RTC0 has 3 CC registers, while RTC1 and RTC2 has 4 CC registers. 
 Your code looks correct to me. This should enable all four CC registers with different values, and interrupts enabled. 
 Best regards Torbjørn

nRF52840 SDK16 - RTC Driver general questions - instance, initialization, and channels

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