MC68336/376
CONFIGURABLE TIMER MODULE 4
MOTOROLA
USER’S MANUAL
Rev. 15 Oct 2000
10-17
During arbitration, the BIUSM provides the arbitration value specified by IARB[2:0] in
BIUMCR and IARB3 in PWMSIC. If the CTM4 wins arbitration, it responds with a vec-
tor number generated by concatenating VECT[7:6] in BIUMCR and the six low-order
bits specified by the number of the submodule requesting service. Thus, for PWMSM8
in the CTM4, the six low-order bits would be eight in decimal, or %00100 in binary.
10.9.8 PWM Frequency
The relationship between the PWM output frequency (fPWM) and the MCU system
clock frequency (fsys) is given by the following equation:
where NCLOCK is the divide ratio specified by the CLK[2:0] field in PWMSIC and
NPERIOD is the period specified by PWMA1.
The minimum PWM output frequency achievable with a specified number of bits of res-
olution for a given system clock frequency is:
where NCPSM is the CPSM divide ratio of two or three.
Similarly, the maximum PWM output frequency achievable with a specified number of
bits of resolution for a given system clock frequency is:
Tables 10-5 and 10-6 summarize the minimum pulse widths and frequency ranges
available from the PWMSM based on the CPSM system clock divide ratio and a
system clock frequency of 20.97 MHz.
Table 10-5 PWM Pulse and Frequency Ranges (in Hz) Using ÷ 2 Option (20.97 MHz)
fsys
Divide
Ratio
Minimum
Pulse
Width
Bits of Resolution
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
÷ 2
0.095
s
160
320
640
1280
2560
5120
10239
20479
40957
81914
164K
328K
655K
1311K
2621K
5243K
÷ 4
0.191
s
80
160
320
640
1280
2560
5120
10239
20479
40957
81914
164K
328K
655K
1311K
2621K
÷ 8
0.381
s
40
80
160
320
640
1280
2560
5120
10239
20479
40957
81914
164K
328K
655K
1311K
÷ 16
0.763
s
20
40
80
160
320
640
1280
2560
5120
10239
20479
40957
81914
164K
328K
655K
÷ 32
1.53
s
10
20
40
80
160
320
640
1280
2560
5120
10239
20479
40957
81914
164K
328K
÷ 64
3.05
s
5
10
20
40
80
160
320
640
1280
2560
5120
10239
20479
40957
81914
164K
÷ 128
6.10
s
2.5
5
10
20
40
80
160
320
640
1280
2560
5120
10239
20479
40957
81914
÷ 512
24.42
s
0.6
1.3
2.5
5
10
20
40
80
160
320
640
1280
2560
5120
10239
20479
f
PWM
f
sys
N
CLOCK
N
PERIOD
-----------------------------------------------
=
Minimum f
PWM
f
sys
256N
CPSM
2
Bits of Resolution
-----------------------------------------------------------------------
=
Maximum f
PWM
f
sys
N
CPSM
2
Bits of Resolution
-------------------------------------------------------------
=