9. Timer#
Over last couple of chapters, we have used sleep_ms() function in different codes. You might have wondered how the μC would have kept track of time. This is commonly done using PWM CTR value. By setting a PWM slice to update the CTR value at 1MHz rate, this would require the DIV value to be 125 for the RP2040 running at 125MHz, we get an accurate 1μs measurement. Note that the CTR value in this case directly represents the time passed since it started counting in μs unit. Many microcontrollers have 32-bit CTR values. However, since the CTR value for RP2040 is 16-bit long, it can measure only 216/1000000 = 0.065536 seconds accurately before CTR overflows. This is quite short for applications that require longer time keeping.
To achieve longer time measurement, RP2040 has a 64-bit CTR register, as part of the Timer peripheral, that updates at a fixed 1MHz update rate. Thus, with this timer, RP2040 can measure time for 264/1000000 = 18446744073709.551616 seconds ≅ 584942 years with 1μs accuracy. Practically, this timer will never overflow, which is why it is also known as a Monotonic Timer. This timer is also the simplest peripheral on RP2040 since all it does is measure time since the μC was started.
Next section provides a list of functions that can be helpful in getting the value of this timer. In my opinion, the library for this peripheral is made unnecessarily complicated since it uses custom datatype, absolute_time_t, at different places to represent time instead of uint64_t. Moreover, it defines multiple functions that use both absolute_time_t and uint64_t as arguments, which makes coding quite confusing. So, please pay attention to the function argument type and its usage.