
MOTOROLA
12-18
CONFIGURABLE TIMER MODULE 7
MC68HC16R1/916R1
USER’S MANUAL
During arbitration, the BIUSM provides the arbitration value specified by IARB[2:0] in
BIUMCR and IARB3 in PWMSIC. If the CTM7 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 CTM7, the six low-order bits would be eight in decimal, or %00100 in binary.
12.10.8 PWM Frequency
The relationship between the PWM output frequency (f
PWM
) and the MCU system
clock frequency (f
sys
) is given by the following equation:
where N
CLOCK
is the divide ratio specified by the CLK[2:0] field in PWMSIC and
N
PERIOD
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 N
CPSM
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
12-6
and
12-7
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 16.78 MHz.
Table 12-6 PWM Pulse and Frequency Ranges (in Hz) Using
÷
2 Option (16.78 MHz)
f
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
0.119
μ
s
128
256
512
1024
2048
4096
8192
16384
32768
65.5k
131k
262k
524k
1049k
2097k
4195k
÷
4
0.238
μ
s
64
128
256
512
1024
2048
4096
8192
16384
32768
65.5k
131k
262k
524k
1049k
2097k
÷
8
0.477
μ
s
32
64
128
256
512
1024
2048
4096
8192
16384
32768
65.5k
131k
262k
524k
1049k
÷
16
0.954
μ
s
16
32
64
128
256
512
1024
2048
4096
8192
16384
32768
65.5k
131k
262k
524k
÷
32
1.91
μ
s
8.0
16
32
64
128
256
512
1024
2048
4096
8192
16384
32768
65.5k
131k
262k
÷
64
3.81
μ
s
4.0
8.0
16
32
64
128
256
512
1024
2048
4096
8192
16384
32768
65.5k
131k
÷
128
7.63
μ
s
2.0
4.0
8.0
16
32
64
128
256
512
1024
2048
4096
8192
16384
32768
65.5k
÷
512
30.5
μ
s
0.5
1.0
2.0
4.0
8.0
16
32
64
128
256
512
1024
2048
4096
8192
16384
f
PWM
f
N
CLOCK
N
PERIOD
-----------------------------------------------
=
Minimum f
PWM
f
256N
CPSM
2
Bits of Resolution
-----------------------------------------------------------------------
=
Maximum f
PWM
f
N
CPSM
2
Bits of Resolution
------------------------------------------------------------
=