ST AN1504 APPLICATION NOTE

AN1504

APPLICATION NOTE

STARTING A PWM SIGNAL DIRECTLY

AT HIGH LEVEL USING THE ST7 1 6-BIT TIMER

by Microcontroller Division Applications

INTRODUCTION

The 16-bit timer is a standard peripheral of the ST7 microcontroller family. This peri pheral can be used for a variety of purposes, including pulse length measurement of up to two input signals (input capture feature) or generation of up to two output waveforms (output compare and PWM mode).

This applicat ion note is abo ut using th e PWM mod e of the st andard 16 bit ti mer. It ex plains how to synchronize the PWM signal output. In other words, how to make s ure it outputs a high state when the counter res tarts after it ha s been stopp ed (for any reas on) or simply when it starts at the beginning of the application. In some applications, lik e motor control, it may be essential to output the high level part of the signal duty cycle when the counter is started.

AN1504/0302 1/9

STARTING A PWM SIGNAL DIRECTLY AT HIGH LEVEL USING THE ST7 16-BIT TIMER

1 16-BIT TIMER PWM MODE

1.1 DESCRIPTION

In pulse width modulation mode, the frequency of the signal is determined by the value in Output Compare 2 register (OC2R) and the pulse length by the value in the Output Compare 1 register (OCR1) or duty cycle value.

The OLVL2 bit selects the level to be applied to the output pin after a successful comparison between the counter and the OC2R register and the OLVL1 bit selects the level to be applied on the output after a successful comparison between the counter and the OC1R register

1.2 NORMAL BEHAVIOUR

Figure 1 shows the normal behaviour of output compare 1 (OCMP1) pin when a PWM signal is output with OLVL2=1 and OLVL1=0.

Figure 1. PWM output when O LVL2=1 and OLVL1=0

FREE RUNNING

FREE RUNNING COUNTER VALUE

COUNTER VALUE

Ttimer × 65535

OLVL1=0

OLVL2= 1

time

FFFFh

FFFFh FFFCh

FFFCh

OC2R

OC2R OC1R

OC1R

0000h

OCMP1

Ouput Compare pin

Ouput Compare pin Timer output

Timer output

Tmax =

When the counter reaches the OC2R register value, the value of OLVL2 is applied on the OCMP1 pin (=1 in this case). W hen the c ounter reaches the v alue of the O C1R register, the value of OLVL1 is applied on the OCMP1 pin (= 0). The formulas needed to compute the values to be put in OC2R and OC1R registers are in the 16- bit Timer chapter of the ST7 datasheets. As the 16-bit Timer is reset at FFFC, the formulas are:

OCiR=((t*Fcpu)/presc)-5 where: t= period of the signal

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STARTING A PWM SIGNAL DIRECTLY AT HIGH LEVEL USING THE ST7 16-BIT TIMER

Fcpu= CPU frequency presc= Timer 16 presc Figure 2 show s the wav ef orms we see on an osc illos cope if th e PWM s ignal is ini tialized a t

10KHz with a 50% duty c ycle (OLVL2=1 and OLVL1=0) and if a flag is set as soon as the counter is started.

Figure 2. OCMP1 waveform with 50% Duty Cycle

PWM OUTPUT

Timer 16 start output enabled flag

We can see from this figure that we have to wait until the counter reaches the OC2R r egister value to get the first high state on the OCMP1 pin. So we have lost one PWM cycle before getting the first high level and in some applications, like energizing motor windings, this is not acceptable. After a motor demagnetization phase, if the windings are not energized immediately when the PWM is started, the motor can stall. The purpose of the following sections is to explain how to get a high level on OCMP1 pin immediately.

To solve the problem, we have to handle two cases.

1. The PWM signal is high when the timer is between F FFC (its reset value) and the OC1R register value. This means OLVL1=0 and OLVL2=1.

2. The PWM signal is high when the timer is between the OC1R register value and the OC2R register value. This means OLVL1=1 and OLVL2=0.

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