ST AN590 Application note

AN590

APPLICATION NOTE

PWM GENERATION WITH ST62 AUTO-RELOAD TIMER

by Microcontroller Division Applications

INTRODUCTION

This note des cribe s how to u se th e ST 62 A uto -re load Tim er ( AR Tim er) to g ene rate a PW M
signal with tunable frequency an d duty cycle. As an examp le, the genera tion of a 31.25 kHz
PWM signal with duty cycle proportional to an analog input voltage is presented.

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 pin. It is 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 m ode if an external event is
present on the input pin. The prescaler ratio can be programmed to choose the timer input fre­quency F

(see Table 1).

IN

Figure 1. Aut o- reload Timer Block Diagr am

AN590/0703 1/7

1

PWM GENER ATION WITH ST62 AUT O- RELOAD TIMER

Pulse Width Modulation (PWM) Generation

Using the PWM gen eration ca pability of the AR -Timer, th e micro contro ller CPU has only to
start/stop the time r and update the duty cycle. High s peed PWM signals in th e range of
100kHz can be generated. The timer clock input frequency
clock f

and the prescaler ratio. The PWM signal period is controlled by the Reload register.

osc

can be selected by the oscillator

IN

F

The duty cycle is defined by the Compare register CP (see Figure 2).
The register TC is incremented from RC to 255d, then reloaded at RC to count again. The

PWM output is set on the overflow of TC and reset when TC=CP.
CP can have any valu e betwe en RC and 2 55d. So RC sho uld be m inimal and the presc aler

ratio as small as possible to achieve a maximum resolution.
Major formulae for PWM generation are:
F

IN

F

PWM

= f

/ (prescaler ratio)

osc

= FIN / (256 - RC)
Duty cycle: (CP -RC) / (256 - RC)
Resolution: 1 / (256 - RC)
With a 8MHz oscillator and a pre scaler ratio of 1, the m aximum resol ution (1/256) leads to a

PWM frequency F

of 31.25kHz. A resolution of 1/64 allows to increase F

PWM

to 125kHz.

PWM

Table 1. Prescaler Programming Ra tio

Bit 0

PS2 PS1 PS0

Reg.SC1

0000 1
0001 2
0010 4
0011 8
0100 16
0101 32
0110 64
0111 128
1000 3
1001 6
1010 12
1011 24
1100 48
1101 96
1110 192
1111 384

PRESCALER

Ratio

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2

Figure 2. PWM Timer Operation

PWM GENERAT ION WITH ST62 AUTO-RELOAD TIMER

Example 1 :

Target: Generate a 12kHz PWM signal with a duty cycle of 37%, using an oscillator frequency
of 4MHz:

We want to generate a periodic signal at a frequency of 12 kHz and a duty cycle of 37% .
The CPU frequency (quartz frequency) is 4 MHz.
Let’s try with a prescaler ratio of 3:

= 4 MHz / 3 = 1333.33 kHz

F

IN

12 kHz = 1333.33 kHz / (256d - RC)
gives RC = 144.889, rounded to 145
Resolution = 1 /(256 - 145) = 1 / 111
Duty cycle desired: 37%

0.37 = (CP - 145) / (256 - 145) yields CP = 186.07, rounded to 186.
Summary: p resc aler = 3 , R C = 14 5d, CP = 18 6d: the se nu mbers yi eld a PW M freque nc y of

12.012 kHz, a duty cycle of 36.94 % and a resolution of 0.9 %, which is very cl ose to the init ial
goal.

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