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R8C/18 Group, R8C/19 Group
14.1.6
Notes on Timer X
•
Timer X stops counting after a reset. Set the values in the timer and prescaler before the count
starts.
•
Even if the prescaler and timer are read out in 16-bit units, these registers are read 1 byte at a
time by the MCU. Consequently, the timer value may be updated during the period when these
two registers are being read.
•
Do not rewrite bits TXMOD0 to TXMOD1, and bits TXMOD2 and TXS simultaneously.
•
In pulse period measurement mode, bits TXEDG and TXUND in the TXMR register can be set to
0 by writing 0 to these bits by a program. However, these bits remain unchanged if 1 is written.
When using the READ-MODIFY-WRITE instruction for the TXMR register, the TXEDG or TXUND
bit may be set to 0 although these bits are set to 1 while the instruction is being executed. In this
case, write 1 to the TXEDG or TXUND bit which is not supposed to be set to 0 with the MOV
instruction.
•
When changing to pulse period measurement mode from another mode, the contents of bits
TXEDG and TXUND are undefined. Write 0 to bits TXEDG and TXUND before the count starts.
•
The TXEDG bit may be set to 1 by the prescaler X underflow generated after the count starts.
•
When using the pulse period measurement mode, leave two or more periods of the prescaler X
immediately after the count starts, then set the TXEDG bit to 0.
•
The TXS bit in the TXMR register has a function to instruct timer X to start or stop counting and a
function to indicate that the count has started or stopped.
0 (count stops) can be read until the following count source is applied after 1 (count starts) is
written to the TXS bit while the count is being stopped. If the following count source is applied, 1
can be read from the TXS bit. After writing 1 to the TXS bit, do not access registers associated
with timer X (registers TXMR, PREX, TX, TCSS, and TXIC) except for the TXS bit, until 1 can be
read from the TXS bit. The count starts at the following count source after the TXS bit is set to 1.
Also, after writing 0 (count stops) to the TXS bit during the count, timer X stops counting at the
following count source.
1 (count starts) can be read by reading the TXS bit until the count stops after writing 0 to the TXS
bit. After writing 0 to the TXS bit, do not access registers associated with timer X except for the
TXS bit, until 0 can be read from the TXS bit.
Rev.1.30
Apr 14, 2006
REJ09B0222-0130
Page 116 of 233
14. Timers
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