2.10.1.3 - Period Measurement (32-Bit, Modes 2 & 3) [UE9 Datasheet]

Mode 2: On every rising edge seen by the external pin, this mode records the number of clock cycles (clock frequency determined by TimerClockBase/TimerClockDivisor) between this rising edge and the previous rising edge. The value is updated on every rising edge, so a read returns the time between the most recent pair of rising edges.

In this 32-bit mode, the Control processor must jump to an interrupt service routine to record the time, so small errors can occur if another interrupt is already in progress. The possible error sources are:

• Other edge interrupt timer modes (2/3/4/5/8/9/12/13). If an interrupt is already being handled due to an edge on another timer, delays of a couple microseconds per timer are possible. That means if five other edge detecting timers are enabled, it is possible (but not likely) to have about 10 microseconds of delay.
• If a stream is in progress, every sample is acquired in a high-priority interrupt. These interrupts could cause delays of up to 10 microseconds.
• The always active UE9 system timer causes an interrupt 11.4 times per second. If this interrupt happens to be in progress when the edge occurs, a delay of about 1 microsecond is possible. If the software watchdog is enabled, the system timer interrupt takes longer to execute and a delay of a few microseconds is possible.

Note that the minimum measurable period is limited by the edge rate limit discussed in Section 2.10.2.

Writing a value of zero to the timer performs a reset. After reset, a read of the timer value will return zero until a new edge is detected. If a timer is reset and read in the same function call, the read returns the value just before the reset.

Mode 3 is the same except that falling edges are used instead of rising edges.