PIC18F2331/2431/4331/4431
The actual dead time is calculated from the DTCON
register as follows:
18.7.4
DEAD-TIME DISTORTION
Note 1: For small PWM duty cycles, the ratio of
dead time to the active PWM time may
become large. In this case, the inserted
dead time will introduce distortion into
waveforms produced by the PWM
module. The user can ensure that
dead-time distortion is minimized by
keeping the PWM duty cycle at least
three times larger than the dead time. A
similar effect occurs for duty cycles at or
near 100%. The maximum duty cycle
used in the application should be chosen
such that the minimum inactive time of
the signal is at least three times larger
than the dead time. If the dead time is
greater or equal to the duty cycle of one
of the PWM output pairs, then that PWM
pair will be inactive for the whole period.
Dead Time = Dead-Time Value/(FOSC/Prescaler)
Table 18-3 shows example dead-time ranges as a
function of the input clock prescaler selected and the
device operating frequency.
TABLE 18-3: EXAMPLE DEAD-TIME
RANGES
FOSC
(MHz)
Prescaler Dead-Time Dead-Time
MIPS
Selection
Min
Max
40
40
40
40
32
32
32
32
25
25
25
25
20
20
20
20
10
10
10
10
5
10
10
10
10
8
FOSC/2
FOSC/4
FOSC/8
FOSC/16
FOSC/2
FOSC/4
FOSC/8
FOSC/16
FOSC/2
FOSC/4
FOSC/8
FOSC/16
FOSC/2
FOSC/4
FOSC/8
FOSC/16
FOSC/2
FOSC/4
FOSC/8
FOSC/16
FOSC/2
FOSC/4
FOSC/8
FOSC/16
FOSC/2
FOSC/4
FOSC/8
FOSC/16
50 ns
100 ns
200 ns
400 ns
62.5 ns
125 ns
250 ns
500 ns
80 ns
3.2 s
6.4 s
12.8 s
25.6 s
4 s
8
8 s
2: Changing the dead-time values in
DTCON when the PWM is enabled may
result in an undesired situation. Disable
the PWM (PTEN = 0) before changing
the dead-time value
8
16 s
8
32 s
6.25
6.25
6.25
6.25
5
5.12 s
10.2 s
20.5 s
41 s
160 ns
320 ns
640 ns
100 ns
200 ns
400 ns
800 ns
200 ns
400 ns
800 ns
1.6 s
400 ns
800 ns
1.6 s
3.2 s
0.5 s
1 s
18.8 Independent PWM Output
Independent PWM mode is used for driving the loads
(as shown in Figure 18-19) for driving one winding of a
switched reluctance motor. A particular PWM output
pair is configured in the Independent Output mode
when the corresponding PMOD bit in the PWMCON0
register is set. No dead-time control is implemented
between the PWM I/O pins when the module is operat-
ing in the Independent PWM mode and both I/O pins
are allowed to be active simultaneously. This mode can
also be used to drive stepper motors.
6.4 s
5
12.8 s
25.6 s
51.2 s
12.8 s
25.6 s
51.2 s
102.4 s
25.6 s
51.2 s
102.4 s
204.8 s
32 s
5
5
2.5
2.5
2.5
2.5
1.25
1.25
1.25
1.25
1
18.8.1
DUTY CYCLE ASSIGNMENT IN THE
INDEPENDENT PWM MODE
5
5
In the Independent PWM mode, each duty cycle gener-
ator is connected to both PWM output pins in a given
PWM output pair. The odd and even PWM output pins
are driven with a single PWM duty cycle generator.
PWM1 and PWM0 are driven by the PWM channel
which uses the PDC0 register to set the duty cycle,
PWM3 and PWM2 with PDC1, PWM5 and PWM4 with
PDC2, and PWM7 and PWM6 with PDC3 (see
Figure 18-3 and Register 18-4).
5
4
4
1
64 s
4
1
2 s
128 s
256 s
4
1
4 s
2010 Microchip Technology Inc.
DS39616D-page 193
MICROCHIP [ MICROCHIP ]
