AN591
Application note
Input capture with ST62 auto-reload timer
Introduction
This note presents how to use the Auto-reload timer (ARTimer) of the ST62 to measure
durations or frequencies of an input signal.
Auto-reload timer description
This timer is an 8 bit timer/counter with prescaler. It includes auto-reload PWM, capture and
compare capability with one input and one output pins. It can be controlled by the following
registers (8 bit):
●
●
●
●
●
●
Mode Control Register (MC)
Status registers (SC0, SC1)
Load register (LR)
Incremental counter register (TC)
Compare register (CP)
Reload/Capture register (RC)
It can also wake-up the MCU from wait mode and exit from stop mode if an external event is
present on the input pin. The prescaler ratio can be programmed to choose the timer input
frequency FIN (see Table 1).
Figure 1.
Auto-reload timer block diagram
July 2008
Rev 2
1/5
www.st.com
Input capture with ST62 auto-reload timer
Capture mode with reset
AN591
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
PS2
0
0
0
0
1
1
1
1
0
0
0
PS1
0
0
1
1
0
0
1
1
0
0
1
PS0
0
1
0
1
0
1
0
1
0
1
0
Prescaler ratio
1
2
4
8
16
32
64
128
3
6
12
Bit 0
Reg. SC1
0
0
0
0
0
0
0
0
1
1
1
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AN591
Programmable example
Input capture with ST62 auto-reload timer
;**************** accumulator *****************************
A .def 0FFh,0FFh,0FFh ;Accumulator
;**************** RAM variable ****************************
period .def 084h,0FFh,0FFh ;elapsed time
;**************** A-R Timer Register Set ******************
RC. def 0D9h,0FFh,0FFh ;reload/capture register
CP. def 0DAh,0FFh,0FFh ;compare register
MC. def 0D5h,0FFh,0FFh ;mode control register
SC0. def 0D6h,0FFh,0FFh ;status/control register 0
SC1. def 0D7h,0FFh,0FFh ;status/control register 1
LR. def 0DBh,0FFh,0FFh ;load register
;==========================================================
ldi SC1,01000100b; clock source = CPU,
;prescaler ratio = 4,
;pull-up on TIMIN disabled,
;capture on rising edge of TIMIN
ldi MC, 11000010b;capture with reset, ints. disabled
;PWMOUT disabled, start timer
; .
; . normal program
; .
;now we want to know the elapsed time between last and
;next edge on TIMIN:
res 2, SC0 ;clear external event flag
wt_edge
jrr 2,SC0,wt_edge ;wait for edge on TIMIN, then
ld A,RC ;read elapsed time from reload/capt
ld period,A ;register into variable ”period”
The following figure shows the evolution of the contents of TC and RC during capture
operation: a rising edge on TIMIN results in copying the latest value of TC into RC, and
clearing TC. Afterwards, TC increments at the selected clock rate until the next capture or
until it reaches 255. In the latter case, it wraps to 0. In capture mode, TC always counts from
0 to 255, it is never loaded from the RC register.
Figure 2.
TC and RC Evolution
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Input capture with ST62 auto-reload timer
AN591
Revision history
Table 2.
Date
March-1993
04-July-2008
Revision history
Revision
1
2
Initial release
Logo modified
Description of changes
4/5
AN591
Input capture with ST62 auto-reload timer
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