LM3S818 Luminary Micro, Inc, LM3S818 Datasheet - Page 148

• Manufacturer Part Number: LM3S818
• Description: Lm3s818 Arm Microcontroller
• Manufacturer: Luminary Micro, Inc

General-Purpose Timers
9.2.3.2
148
Table 9-1. 16-Bit Timer with Prescaler Configurations
a. T
16-Bit Input Edge Count Mode
In Edge Count mode, the timer is configured as a down-counter 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 set to 0. The type of edge that the timer counts is determined
by the TnEVENT fields of the GPTMCTL register. During initialization, the GPTM Timern Match
(GPTMTnMATCHR) register is configured so that the difference between the value in the
GPTMTnILR register and the GPTMTnMATCHR register equals the number of edge events that
must be counted.
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 the counter by 1 until the event
count matches GPTMTnMATCHR. When the counts match, the GPTM asserts the CnMRIS bit in
the GPTMRIS register (and the CnMMIS bit, if the interrupt is not masked). The counter is then
reloaded using the value in GPTMTnILR, and stopped since 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.
Figure 9-2 shows how input edge count mode works. In this case, the timer start value is set to
GPTMnILR=0x000A and the match value is set to GPTMnMATCHR=0x0006 so that four edge
events are counted. The counter is configured to detect both edges of the input signal.
Note that the last two edges are not counted since the timer automatically clears the TnEN bit after
the current count matches the value in the GPTMnMR register.
00000000
00000001
00000010
C
Prescale
11111100
------------
11111110
11111111
is the clock period.
#Clock (T
254
255
256
--
1
2
3
C
)
a
Preliminary
Max Time
332.9229
334.2336
335.5443
1.3107
2.6214
3.9321
Units
mS
mS
mS
mS
mS
mS
February 6, 2007