MC68HC05B6_13 Datasheet, PDF (67/302 Pages) Freescale Semiconductor, Inc – Microcontrollers

5.4
Output compare
‘Output compare’ is a technique which may be used, for example, to generate an output waveform,
or to signal when a specific time period has elapsed, by presetting the output compare register to
the appropriate value.
There are two output compare registers: output compare register 1 (OCR1) and output compare
register 2 (OCR2), both of which are read or write registers.
Note:
The same output compare interrupt enable bit (OCIE) is used for the two output
compares.
5
5.4.1 Output compare register 1 (OCR1)
Output compare high 1
Output compare low 1
Address bit 7
bit 6
bit 5
bit 4
bit 3
bit 2
bit 1
bit 0
State
on reset
$0016
Undefined
$0017
Undefined
The 16-bit output compare register 1 is made up of two 8-bit registers at locations $16 (MSB) and
$17 (LSB). The contents of the output compare register 1 are compared with the contents of the
free-running counter continually and, if a match is found, the corresponding output compare flag
(OCF1) in the timer status register is set and the output level (OLVL1) is transferred to pin TCMP1.
The output compare register 1 values and the output level bit should be changed after each
successful comparison to establish a new elapsed timeout. An interrupt can also accompany a
successful output compare provided the corresponding interrupt enable bit (OCIE) is set. (The
free-running counter is updated every four internal bus clock cycles.)
After a processor write cycle to the output compare register 1 containing the MSB ($16), the output
compare function is inhibited until the LSB ($17) is also written. The user must write both bytes
(locations) if the MSB is written first. A write made only to the LSB ($17) will not inhibit the compare
1 function. The processor can write to either byte of the output compare register 1 without affecting
the other byte. The output level (OLVL1) bit is clocked to the output level register and hence to the
TCMP1 pin whether the output compare flag 1 (OCF1) is set or clear. The minimum time required
to update the output compare register 1 is a function of the program rather than the internal
hardware. Because the output compare flag 1 and the output compare register 1 are not defined
at power on, and not affected by reset, care must be taken when initializing output compare
functions with software. The following procedure is recommended:
– Write to output compare high 1 to inhibit further compares;
– Read the timer status register to clear OCF1 (if set);
– Write to output compare low 1 to enable the output compare 1 function.
MC68HC05B6
Rev. 4.1
PROGRAMMABLE TIMER
Freescale
5-9