Freescale Semiconductor, Inc.
Capture/Compare Timer
Reading TRH returns the current value of the high byte of the counter
and causes the low byte to be latched into a buffer, as shown in
Figure 8-8. The buffer value remains fixed even if the high byte is read
more than once. Reading TRL reads the transparent low byte buffer and
completes the read sequence of the timer registers.
INTERNAL DATA BUS
7
7
0
0
LATCH
8
LOW BYTE BUFFER
15
$0018
TIMER REGISTER HIGH
READ TRH
TIMER REGISTER LOW
$0019
Figure 8-8. Timer Register Reads
NOTE: To prevent interrupts from occurring between readings of TRH and TRL,
set the interrupt mask (I bit) in the condition code register before reading
TRH, and clear the mask after reading TRL.
8.4.4 Alternate Timer Registers
The alternate timer registers (ATRH and ATRL) shown in Figure 8-9
contain the current high and low bytes of the 16-bit counter. Reading
ATRH before reading ATRL causes ATRL to be latched until ATRL is
read. Reading does not affect the timer overflow flag (TOF). Writing to
the alternate timer registers has no effect.
Technical Data
98
MC68HC705C8A — Rev. 3
Capture/Compare Timer
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