MC68HC711PH8 Motorola, MC68HC711PH8 Datasheet - Page 171

• Manufacturer Part Number: MC68HC711PH8
• Description: High-density Complementary Metal Oxide Semiconductor (HCMOS) Microcomputer Unit
• Manufacturer: Motorola

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:
Note:
The recommended procedure for configuring timers B and C is as follows:
8.3.2
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.
Warning: Selecting EXTALi as a clock source when the PLL function is not used, i.e. when the
MC68HC11PH8
CSA[2:0]
0 0 0
0 0 1
0 1 0
0 1 1
1 0 0
1 0 1
1 1 0
1 1 1
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.
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).
The timer preset bits only have an effect if the timer is stopped under hardware control
(timers B and C only).
bus frequency is EXTALi/4, could lead to read or write errors in the timer registers.
Timer A clock source
Clock rate selection
EXTALi/8
EXTALi/4
EXTALi/8
EXTALi/8
EXTALi/8
EXTALi/8
EXTALi/8
EXTALi
Table 8-6 Modulus timers clock sources
CSB[2:0] Timer B clock source
0 0 0
0 0 1
0 1 0
0 1 1
1 0 0
1 0 1
1 1 0
1 1 1
TIMING SYSTEM
Timer A underßow
Timer C underßow
Rising edge PH7
EXTALi/4
EXTALi/8
Stopped
Stopped
EXTALi
CSC[2:0] Timer C clock source
0 0 0
0 0 1
0 1 0
0 1 1
1 0 0
1 0 1
1 1 0
1 1 1
Timer A underßow
Timer B underßow
Rising edge PH6
EXTALi /4
EXTALi /8
Stopped
Stopped
EXTALi
MOTOROLA
TPG
8-37
8