ST9 datasheet(81/146 Pages) STMICROELECTRONICS

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ST9 USER GUIDE

This value of the Load Register determines the frequency of the output signals. For example,

with a reload value of 255 and a 25 Mhz internal clock, the PWM frequency is 1/256 of the 25

MHz clock divided by 3, that is 32,552 kHz. In this case, the range of the compare registers is

1 to 255 inclusive. A value of 0 locks the outputs to the high state.

To use the timer to deliver variable PWM, we need to change the value of the compare regis-

ters from time to time. The simple way that first comes to mind is to write into the compare reg-

ister whenever we need to update the PWM rate. This can lead to problems.

If we write the new value into the compare register while the counter value matches the pre-

vious contents, this can make the comparison fail and the pulse that changes the state of the

output is not produced. The result is the output signal misses one cycle, as follows:

Figure 42

. Compare Register is modified while its Content matches Counter

This will obviously have a bad effect on the electronic circuit connected to this output.

Another problem can occur if the reload value exceeds 255. Then, two bytes are needed to ex-

press the compare values. Since the ST9 is an 8-bit machine, the two bytes that make up the

word are sent one after the other. If the high bytes of the old and new values are the same,

there is no problem. But if they differ, there are three possibilities for the value in the compare

– The register contains the old value

– The register contains one byte of the old value, and the other byte of the new value

– The register contains the new value

Reload

Value

Compare

Value

Output A

This pulse is missing.

This comparison has failed

ST9 Datasheet(PDF) 81 Page - STMicroelectronics