Microseconds timer using RTC

Question

Hi everyone, 
 I need to set up a timer using the RTC. The default function that converts ms to ticks APP_TIMER_TICKS() accepts only milliseconds. So, I wrote my own conversion function as below 
 #define APP_TIMER_TICKS_US(US) \
 ((uint32_t)ROUNDED_DIV( \
 (US) * (uint64_t)APP_TIMER_CLOCK_FREQ, \
 1000000 * (APP_TIMER_CONFIG_RTC_FREQUENCY + 1))) 
 and now I can set up a timer in microseconds like this: 
 #define SAADC_INTERVAL APP_TIMER_TICKS_US(200) 
 My questions are: 
 1. Is this approach correct? 
 2. RTC running on 32768Hz meaning that the resolution is ~30us. Does this mean that I can set up a timer down to 30us? For example 
 #define SAADC_INTERVAL APP_TIMER_TICKS_US(30) 
 
 Thanks in advance 
 Nick

Karl Ylvisaker · Accepted Answer

Hello again Nick, 
 Nikosant03 said: I am wondering if it is better to set it at lower priority such as level 2 (I am using nRF52840) or leave as it is... What is your opinion? 
 This depends on what the timer is connected to - how critical is it that it happens on-time, and how long does the connected event take to handle? An exempt from the priority level documentation I linked reads: As seen from Exception model , the application has available priority level 2 and 3, located between the higher and lower priority levels reserved by the SoftDevice. This enables a low-latency application interrupt to support fast sensor interfaces. An application interrupt at priority level 2 or 3 will only experience latency from SoftDevice interrupts at priority levels 0 and 1 , while application interrupts at priority levels 5, 6, or 7 can experience latency from all SoftDevice priority levels. For most use cases, the lower levels 5, 6, and 7 is acceptable, and gives leeway to prioritize certain application tasks before others. IN other words; if there is no explicit need to change the priority to ensure that the interrupt is prioritized over all other application activity, then you should be fine leaving it at the default value - or at most changing it up or down a single level. If you are interfacing with a high-frequency sensor or interface, then the issue might also be alleviated by making use of the PPI and easyDMA features. However, it is hard for me to say anything more certain regarding interrupt priorities without knowing more of the context. Please do not hesitate to ask if you should encounter any other questions or issues! :) Best regards, Karl

Microseconds timer using RTC

Top Replies