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PIC18F1230/1330

2009 Microchip Technology Inc.

DS39758D-page 137

The actual dead time is calculated from the DTCON

Dead Time = Dead-Time Value/(FOSC/Prescaler)

Table 14-3 shows example dead-time ranges as a

function of the input clock prescaler selected and the

device operating frequency.

TABLE 14-3:

EXAMPLE DEAD-TIME

RANGES

14.7.4

DEAD-TIME DISTORTION

14.8

Independent PWM Output

Independent PWM mode is used for driving the loads

(as shown in Figure 14-19) that drive one winding of a

switched reluctance motor. A particular PWM output

pair is configured in the Independent Output mode

when the corresponding PMODx bit in the PWMCON0

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.

14.8.1

DUTY CYCLE ASSIGNMENT IN THE

INDEPENDENT PWM MODE

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 the 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, and PWM5 and PWM4

with PDC2 (see Figure 14-3 and Register 14-3).

FOSC

(MHz)

MIPS

Prescaler

Selection

Dead-Time

Min

Dead-Time

Max

FOSC/2

50 ns

3.2

FOSC/4

100 ns

6.4

FOSC/8

200 ns

12.8

FOSC/16

400 ns

25.6

FOSC/2

62.5 ns

FOSC/4

125 ns

FOSC/8

250 ns

FOSC/16

500 ns

6.25

FOSC/2

80 ns

5.12

6.25

FOSC/4

160 ns

10.2

6.25

FOSC/8

320 ns

20.5

6.25

FOSC/16

640 ns

FOSC/2

100 ns

6.4

FOSC/4

200 ns

12.8

FOSC/8

400 ns

25.6

FOSC/16

800 ns

51.2

2.5

FOSC/2

200 ns

12.8

2.5

FOSC/4

400 ns

25.6

2.5

FOSC/8

800 ns

51.2

2.5

FOSC/16

1.6

102.4

51.25

FOSC/2

400 ns

25.6

51.25

FOSC/4

800 ns

51.2

51.25

FOSC/8

1.6

102.4

51.25

FOSC/16

3.2

204.8

FOSC/2

0.5

s32 s

FOSC/4

s64 s

FOSC/8

s128 s

FOSC/16

s256 s

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 mod-

ule. 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.

Changing the dead-time values in

DTCON when the PWM is enabled may

result in an undesirable situation. Disable

the PWM (PTEN = 0) before changing the

dead-time value.

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