Timer Counter and Capture Registers

The UTMR module has two registers to access the timer/counter value – TUxyTMR counter register and TUxyCR capture register. The size of these registers is the same as the size of the timer. Both registers share the same memory location and are addressed based on theRDSEL bit in the TUxyCON0 register. Setting the RDSEL bit addresses the TUxyTMR counter register, whereas clearing the RDSEL bit addresses the TUxyCR capture register.

To read the raw counter value using the TUxyTMR counter register, the RDSEL bit must be set. When the timer is running in either Synchronous or Asynchronous mode, directly reading the TUxyTMR counter register can produce erroneous values. This can occur when the counter/timer is operating from an asynchronous clock source or when the read happens coincidentally with the rollover of the bottom 8 bits of the TUxyTMR counter register.

Clearing the RDSEL bit directs all counter/timer reads through the TUxyCR capture register. The TUxyCR capture register is functionally a read-only register and is loaded directly from the counter/timer in response to either of the following three conditions:

  1. 1.Setting the CAPT command bit.
  2. 2.When a stop event is generated.
  3. 3.In the event of an ERS rising edge (or falling edge based on EPOL bit selection) if the Stop condition is set to none. See Stop Event for more details on Stop condition.
It is recommended that any read of the timer, when it is running, utilizes the CAPT command bit with the RDSEL bit clear. Asserting the CAPT bit will cause synchronous transfer of the timer value to the TUxyCR capture register. The CAPT bit remains set until the capture is complete. The TUxyCR capture register can then be read by the processor without any data corruption. See Figure 1 for an example of the CAPT bit operation.
Figure 1. CAPT Bit Operation

In the event of an ERS rising capture, the TUxyCR capture register must be read before the event of a second ERS rising or the data captured will be overwritten by the second rising event.

The TUxyTMR counter register can be written when the RDSEL bit is set, provided that the ON bit is clear. Attempting to write to the TUxyTMR counter register with the ON bit set can result in corrupted data. If the intention is to clear the counter, the CLR command bit needs to be used instead of writing zeros. Asserting the CLR bit clears the TUxyTMR counter register, even if the ON bit is set. The CLR bit remains set until the counter is reset.
The CAPT and CLR command bits are subject to synchronization delays which is dependent on the settings of CSYNC and ON bits, as shown in Table 1.
Table 1. Behavior of CAPT and CLR Commands with Respect to ON and CSYNC Bits
ON Bit CSYNC Bit Behavior of CAPT and CLR Commands
1 (Timer Running) 1 Synchronization delay of three timer clock cycles applies before the desired action is performed
1 (Timer Running) 0 No synchronization delay applies. Desired action is performed immediately.
0 (Timer Stopped) 1 Synchronization delay of three timer clock cycles applies. The desired action is delayed until timer clock resumes.
0 (Timer Stopped) 0 No synchronization delay applies. Desired action is performed immediately.
  1. 1.Reading and writing the TUxyTMR counter register when the timer is running (ON = 1) is not recommended. The TUxyTMR counter register needs to be read or written to only when the timer is stopped (ON = 0) to prevent data corruption.
  2. 2.The TUxyTMR register, like many othe registers in the module, remains unchanged after a non-POR/BOR system Reset. It is recommended to always clear this register at the start of program execution to avoid counting from an unknown value.
  3. 3.Setting the CLR bit does not reset the TUxyCR capture register.
  4. 4.The TUxyTMR register needs to not be written as a means to change the effective period. If the intention is to change the timer period, the TUxyPR period register needs to be changed instead. See Timer Period Register for more details on how to change the timer period while the timer is running.
  5. 5.When software sets a CLR or CAPT command bit, the bit value of ‘1’ is indicated in the SFR immediately, to indicate that the over-and-back clock synchronization is not complete. However, a sufficiently high timer clock frequency might complete the cross-domain synchronization within one instruction cycle and the bit value would always appear to be ‘0’.
  6. 6.Setting CLR or CAPT command bits to ‘0’ has no effect.
  7. 7.The timer starts counting by incrementing the TUxyTMR value to the next valid counter value. For instance, if the counter is in Reset state (TUxyTMR = 0), then the timer starts counting from 1. If the TUxyTMR = PR and RESET = at PR Match, then the timer will start counting by resetting the counter to zero first.