Input capture with ST62 auto-reload timer

AN591

2/5

Capture mode with reset

This can be used to measure time durations or frequencies.This mode is used to measure

the time elapsed between two edges of an external signal: two rising edges, two falling

edges, or one rising and one falling if the configuration of the timer is modified after the first

edge detected.

The minimum duration of one signal to measure depends on the CPU clock and on the

required precision. With a 8MHz Quartz crystal, a signal of 8ms duration can be measured

with a resolution of 1/64.

Example:

Let’s measure the time elapsed between two rising edges on TIMIN:

The prescaler ratio must be programmed according to the expected duration to measure. In

this example it is programmed to: prescaler ratio = 4, clock source = CPU clock. (in case the

duration expected can be longer than the 255 count span of the TC count register, the

overflow interrupt can be used to increment a RAM variable, used as the most significant

part of the result).

In the program example, waiting for the edge on TIMIN is made by a software loop (jrr 2,

SC0, wt_edge), but it is also possible to use the external event interrupt to read the result

immediately after the edge without loading the rest of the program. It is also possible to read

the result directly from RC without waiting or using the interrupt, if the duration is required at

an asynchronous time, not just after the edge.

Table 1.

Prescaler programming ratio

Bit 0

Reg. SC1

PS2

PS1

PS0

Prescaler ratio

000

01

000

12

001

04

001

18

010

0

16

010

1

32

011

0

64

011

1

128

100

03

100

16

101

0

12