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Functional Description
1260
SLAU723A – October 2017 – Revised October 2018
Copyright © 2017–2018, Texas Instruments Incorporated
General-Purpose Timers
18.3.3.3 Input Edge-Count Mode
NOTE:
For rising-edge detection, the input signal must be high for at least two clock periods
following the rising edge. Similarly, for falling-edge detection, the input signal must be low for
at least two clock periods following the falling edge. Based on this criteria, the maximum
input frequency for edge detection is 1/4 of the frequency.
In Edge-Count mode, the timer is configured as a 24-bit up- or down-counter including the optional
prescaler with the upper count value stored in the GPTM Timer n Prescale (GPTMTnPR) register and the
lower bits in the GPTMTnR register. In this mode, the timer is capable of capturing three types of events:
rising edge, falling edge, or both. To place the timer in Edge-Count mode, the TnCMR bit of the
GPTMTnMR register must be cleared. The type of edge that the timer counts is determined by the
TnEVENT fields of the GPTMCTL register. During initialization in down-count mode, the
GPTMTnMATCHR and GPTMTnPMR registers are configured so that the difference between the value in
the GPTMTnILR and GPTMTnPR registers and the GPTMTnMATCHR and GPTMTnPMR registers equals
the number of edge events that must be counted. In up-count mode, the timer counts from 0x0 to the
value in the GPTMTnMATCHR and GPTMTnPMR registers. When executing an up count, that the value
of GPTMTnPR and GPTMTnILR must be greater than the value of GPTMTnPMR and GPTMTnMATCHR.
lists the values that are loaded into the timer registers when the timer is enabled.
Table 18-6. Counter Values When the Timer is Enabled in Input Edge-Count Mode
Register
Count Down Mode
Count Up Mode
GPTMTnR
GPTMTnPR in combination with GPTMTnILR
0x0
GPTMTnV
GPTMTnPR in combination with GPTMTnILR
0x0
When software writes the TnEN bit in the GPTM Control (GPTMCTL) register, the timer is enabled for
event capture. Each input event on the CCP pin decrements or increments the counter by 1 until the event
count matches GPTMTnMATCHR and GPTMTnPMR. When the counts match, the GPTM asserts the
CnMRIS bit in the GPTM Raw Interrupt Status (GPTMRIS) register, and holds it until it is cleared by
writing the GPTM Interrupt Clear (GPTMICR) register. If the capture mode match interrupt is enabled in
the GPTM Interrupt Mask (GPTMIMR) register, the GPTM also sets the CnMMIS bit in the GPTM Masked
Interrupt Status (GPTMMIS) register. In up-count mode, the current count of the input events is held in
both the GPTMTnR and GPTMTnV registers. In down-count mode, the current count of the input events
can be obtained by subtracting the GPTMTnR or GPTMTnV from the value made up of the GPTMTnPR
and GPTMTnILR register combination.
In addition to generating interrupts, an ADC and/or a µDMA trigger can be generated. The ADC trigger is
enabled by setting the TnOTE bit in GPTMCTL and the event that activates the ADC is configured in the
GPTM ADC Event (GPTMADCEV) register. The µDMA trigger is enabled by configuring and enabling the
appropriate µDMA channel as well as the type of trigger enable in the GPTM DMA Event (GPTMDMAEV)
register (see
After the match value is reached in down-count mode, the counter is then reloaded using the value in
GPTMTnILR and GPTMTnPR registers, and stopped because the GPTM automatically clears the TnEN
bit in the GPTMCTL register. Once the event count has been reached, all further events are ignored until
TnEN is re-enabled by software. In up-count mode, the timer is reloaded with 0x0 and continues counting.
shows how Input Edge-Count mode works. In this case, the timer start value is set to
GPTMTnILR = 0x000A and the match value is set to GPTMTnMATCHR = 0x0006 so that four edge
events are counted. The counter is configured to detect both edges of the input signal.
The last two edges are not counted because the timer automatically clears the TnEN bit after the current
count matches the value in the GPTMTnMATCHR register.