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The last two possible values for the timer prescaler bits (110 and 111) are reserved and cannot be used to lengthen the counter interval. Unlike the HC11, in which the prescaler control bits were timed-write-once bits, the prescaler bits of the HC12 can be changed during program execution. However, a change in the prescale factor will not take effect until the prescale counter stages reach zero, so the range of possible values of this delay must be considered when writing code that changes the prescale factor during execution.

Note that in the TFLG register, setting a bit (writing a one to that bit) causes the corresponding but to be cleared.