ST9 datasheet(82/146 Pages) STMICROELECTRONICS

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ST9 USER GUIDE

Depending on the relative values of the old and new values, on the order of writing the bytes

(high byte first, or low byte first), and on the direction of the timer (up-counting or down-

counting), various things may happen. These can range from a missed comparison (see

Figure 42) to two comparisons in the same cycle (not a problem).

This indicates that you have to pay attention to the time at which the compare register has to

be updated. If we consider that the duty cycle does not vary widely from one cycle to another,

the safest time to change value is as soon as possible after the match. The best way to do it

is to store the new value in a variable (a register is a good choice), and configure the MFT to

trigger an interrupt when the match occurs. Then, the interrupt service routine is executed and

the new value is safely copied into the compare register.

4.4.2 Generating a Pulse Width Modulated Wave with a Cleaner Spectrum

The use of a MFT to produce two PWM signals is convenient, especially for example in a

stepper motor application. There, an ST92F150 is used and only one MFT is available for

PWM generation, since the other one is used for other purposes. However, this method suf-

fers from a parasitic phase modulation of the signal since the falling edge is fixed, and the

rising edge moves with the desired duty cycle. This leads to the production of unwanted har-

monics in the resulting spectrum. This is not a problem for a lot of applications.

Sometime these harmonics are prohibited. Example: when we want to drive a high power in-

duction motor, these harmonics have two serious drawbacks:

– They produce parasitic frequencies that are injected in the mains, which is not allowed by

power distribution companies,

– They feed the induction motor with even ranked harmonics, which degrade the efficiency of

the motor and reduce the peak output power.

You can improve this situation by using a symmetrical PWM generation, producing waveforms

as follows:

Figure 43

. Symmetrical PWM Waveforms

You use the MFT in the following way. One MFT being used for one output signal, you use

only output A. You configure it so that it is set on a comparison with compare register 1, and

reset on a comparison with compare register 0. These registers are always loaded with values

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