Table 4-8 System Frequencies from 32.768-kHz Reference (Continued)
To obtain clock frequency in kilohertz, find counter modulus in the left column, then look in appropriate prescaler cell.
Shaded cells contain values that exceed specified maximum system frequency.
Modulus
Y
Prescaler
[W:X] = 00
6029
6160
6291
6423
6554
6685
6816
6947
7078
7209
7340
7471
7602
7733
7864
7995
8126
8258
8389
[W:X] = 01
12059
12321
12583
12845
13107
13369
13631
13894
14156
14418
14680
14942
15204
15466
15729
15991
16253
16515
16777
[W:X] = 10
24117
24642
25166
25690
26214
26739
27263
27787
28312
28836
29360
2988
[W:X] = 11
48234
49283
50332
51380
52428
53477
54526
55575
56623
57672
58720
59769
60817
61866
62915
63963
65011
66060
67109
101101
101110
101111
110000
110001
110010
110011
110100
110101
110110
110111
111000
111001
111010
111011
111100
111101
111110
111111
30409
30933
31457
31982
32506
33030
33554
4
4.3.3 External Bus Clock
The state of the external clock division bit (EDIV) in SYNCR determines clock rate for
the external bus clock signal (ECLK) available on pin ADDR23. ECLK is a bus clock
for MC6800 devices and peripherals. ECLK frequency can be set to system clock fre-
quency divided by eight or system clock frequency divided by sixteen. The clock is en-
abled by the CS10 field in chip select pin assignment register 1 (CSPAR1). ECLK
operation during low-power stop is described in the following paragraph. Refer to 4.8
Chip Selects for more information about the external bus clock.
4.3.4 Low-Power Operation
Low-power operation is initiated by the CPU32. To reduce power consumption selec-
tively, the CPU can set the STOP bits in each module configuration register. To mini-
mize overall microcontroller power consumption, the CPU can execute the LPSTOP
instruction, which causes the SIM to turn off the system clock.
When individual module STOP bits are set, clock signals inside each module are
turned off, but module registers are still accessible.
When the CPU executes LPSTOP, a special CPU space bus cycle writes a copy of
the current interrupt mask into the clock control logic. The SIM brings the MCU out of
low-power operation when either an interrupt of higher priority than the stored mask or
a reset occurs. Refer to 4.5.4.2 LPSTOP Broadcast Cycle and SECTION 5 CEN-
TRAL PROCESSING UNIT for more information.
MC68331
SYSTEM INTEGRATION MODULE
MOTOROLA
4-15
USER’S MANUAL
MOTOROLA [ MOTOROLA ]
