LM3S2637 Datasheet, PDF (332/678 Pages) Texas Instruments – Stellaris® LM3S2637 Microcontroller

General-Purpose Timers
Figure 9-2. 16-Bit Input Edge Count Mode Example
Count
Timer stops,
flags
asserted
Timer reload
on next cycle Ignored
Ignored
0x000A
0x0009
0x0008
0x0007
0x0006
Input Signal
9.3.3.3
16-Bit Input Edge Time Mode
Note:
For rising-edge detection, the input signal must be High for at least two system clock periods
following the rising edge. Similarly, for falling edge detection, the input signal must be Low
for at least two system clock periods following the falling edge. Based on this criteria, the
maximum input frequency for edge detection is 1/4 of the system frequency.
Note: The prescaler is not available in 16-Bit Input Edge Time mode.
In Edge Time mode, the timer is configured as a free-running down-counter initialized to the value
loaded in the GPTMTnILR register (or 0xFFFF at reset). The timer is capable of capturing three
types of events: rising edge, falling edge, or both. The timer is placed into Edge Time mode by
setting the TnCMR bit in the GPTMTnMR register, and the type of event that the timer captures is
determined by the TnEVENT fields of the GPTMCTL register.
When software writes the TnEN bit in the GPTMCTL register, the timer is enabled for event capture.
When the selected input event is detected, the current Tn counter value is captured in the GPTMTnR
register and is available to be read by the controller. The GPTM then asserts the CnERIS bit (and
the CnEMIS bit, if the interrupt is not masked).
After an event has been captured, the timer does not stop counting. It continues to count until the
TnEN bit is cleared. When the timer reaches the 0x0000 state, it is reloaded with the value from the
GPTMTnILR register.
Figure 9-3 on page 333 shows how input edge timing mode works. In the diagram, it is assumed that
the start value of the timer is the default value of 0xFFFF, and the timer is configured to capture
rising edge events.
Each time a rising edge event is detected, the current count value is loaded into the GPTMTnR
register, and is held there until another rising edge is detected (at which point the new count value
is loaded into GPTMTnR).
332
June 18, 2012
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