R8C/1A Group, R8C/1B Group 14. Timers
Rev.1.30 Dec 08, 2006 Page 122 of 315
REJ09B0252-0130
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.
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