The compare control register also contains a SHADOW bit. It is normally cleared, but it can be set this way:

TimerRegs.T16PWM0CMPCTRL.bit.SHADOW = 1;

The SHADOW bit is useful for a PWM output that will vary over time. The compare blocks work by detecting when the compare data is equal to the 16 bit counter value. This works fine if the compare data is never changed.

If, however, the compare data value is reduced at the wrong time, it is possible for the compare block to miss it, and therefore for the PWM pulse to remain on for the entire period.

For example, suppose that the compare 1 data value is 100. Suppose that the counter value is 90, and the compare 1 data register is written to, changing the data value to 80. In this case, for one cycle, the compare 1 event will not occur. Normally for PWM, compare 1 is used to turn off the PWM pulse. So the result will be a PWM pulse that is on for the entire period.

If the SHADOW bit is set, it enables buffer registers, called shadow registers, for the compare data registers. The data written is stored in the shadow register until the next compare event, and only then is it written into the compare data register.

So to return to the example, the 80 will not be written into the data register until after the counter reaches 100, so the pulse will stop at 100, instead of going on to the end.

There are two side effects to using the shadow register:

1. Read always reads from the actual data register. So if the data register is written to and then read from before the next compare event for that block, the old data will still be read.
2. If the value written to the data register is higher than the current value, it may be possible to have two compare events. For example if the counter value is 70, the data value is 80, and the new data value is 100, the compare event will occur at 80, and the shadow register will be copied into the data register. Then a second compare event will occur when the 16 bit counter reaches 100. This should not be a problem if the compare action is a set or reset, but if the action is a toggle, this could be a problem. Or if the compare interrupt is being used.

Of course, the same thing could happen without the shadow bit set.