MC68HC11PH8 Datasheet, PDF (171/264 Pages) Motorola, Inc – High-density Complementary Metal Oxide Semiconductor (HCMOS) Microcomputer Unit

The modulus register can be written to at any time without affecting the down-counter; care must
be taken by the programmer to ensure that a new value is written to the modulus register before
the down-counter reaches $00, unless the previous value is to be reloaded. A read of the modulus
register with the timer running will access a latched value of the down-counter. This latch
guarantees a stable value during the read and is updated with each timer clock. To determine the
value in the modulus register itself (for timers B and C), the timer must be stopped by the
appropriate clock selection (see Section 8.3.2), then a one must be written to the preset bit in the
control register. The down-counter now contains the modulus register value.
Note:
Because it is used to clock the COP watchdog in applications using PLL clock
generation, timer A cannot be stopped (therefore, the value in modulus register A
cannot be determined).
Note:
The timer preset bits only have an effect if the timer is stopped under hardware control
(timers B and C only).
The recommended procedure for configuring timers B and C is as follows:
1) Stop the timer by writing %000 to the relevant clock select control bits.
2) Set the modulus register by writing the required value to the timer data register.
3) Write a one to the timer preset bit.
4) Select the desired clock source to start the counter decrementing.
8
8.3.2 Clock rate selection
A number of clock rates can be selected in software for each of the three modulus timers. The
selection is controlled by bits 0-2 in the timer control register, as shown in the tables below.
Timer A may be used as a prescaler for timers B and C (see Figure 8-7). Similarly timer B may be
used as a prescaler for timer C, and vice versa.
Table 8-6 Modulus timers clock sources
CSA[2:0]
000
001
010
011
100
101
110
111
Timer A clock source
EXTALi/8
EXTALi
EXTALi/4
EXTALi/8
EXTALi/8
EXTALi/8
EXTALi/8
EXTALi/8
CSB[2:0] Timer B clock source
000
Stopped
001
EXTALi
010
EXTALi/4
011
EXTALi/8
1 0 0 Timer A underßow
1 0 1 Timer C underßow
110
Rising edge PH7
111
Stopped
CSC[2:0] Timer C clock source
000
Stopped
001
EXTALi
010
EXTALi /4
011
EXTALi /8
1 0 0 Timer A underßow
1 0 1 Timer B underßow
1 1 0 Rising edge PH6
111
Stopped
Warning: Selecting EXTALi as a clock source when the PLL function is not used, i.e. when the
bus frequency is EXTALi/4, could lead to read or write errors in the timer registers.
MC68HC11PH8
TIMING SYSTEM
TPG
MOTOROLA
8-37