ST AN591 Application note

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 AN591

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

00001

00012

00104

00118

010016

010132

011064

0111128

10003

10016

101012

PS2 PS1 PS0 Prescaler ratio

2/5