c5502 timer to measure period

Hi Christian,

You can measure the period of your square wave with two timers: TIMER 0 that acts as a counter and TIMER 1 that acts as a GPIO input on the TIM1 pin. You can configure the GPIO input to interrupt on the rising edge of an external signal (your square wave). (See 3.11 Timer Operation in the GPIO Mode in the VC5501/5502 DSP Timers Reference Guide)

When the timer pin sees a rising edge, an interrupt will fire. Inside of the interrupt service routine (ISR), you can calculate the time elapsed since the previous rising edge by subtracting the values of the TIMER 0 counter read during the previous ISR from the value read during the current ISR. (See 3.11.3 Timer Pin Used as Input to Source the Interrupt and 3.12.7 Reading the Counter Registers in the VC5501/5502 DSP Timers Reference Guide)

The time elapsed between rising edges is the period of your external signal. You may want to average several period measurements to account for jitter.

For precision, you should set up TIMER 0 to increment its counter at a rate much faster than the frequency of the external signal.

The voltage of external signal needs to be within the recommended operating conditions for the device. (See 5.3 Recommended Operating Conditions in the VC5502 Datasheet)

Don’t forget to configure the Timer Signal Selection Register (TSSR) to use the TIM1 pin as the timer input pin. (See 3.5.3 Timer Signal Selection Register (TSSR) in the VC5502 Datasheet)

Hope this helps,

Mark