PIC16F684 datasheet(87/164 Pages) MICROCHIP

87 / 164 page

 2004 Microchip Technology Inc.

Preliminary

DS41202C-page 85

PIC16F684

11.3.6

PROGRAMMABLE DEAD BAND

DELAY

In half-bridge applications where all power switches are

modulated at the PWM frequency at all times, the

power switches normally require more time to turn off

than to turn on. If both the upper and lower power

switches are switched at the same time (one turned on,

and the other turned off), both switches may be on for

a short period of time until one switch completely turns

off. During this brief interval, a very high current

(shoot-through current) may flow through both power

switches, shorting the bridge supply. To avoid this

potentially destructive shoot-through current from

flowing during switching, turning on either of the power

switches is normally delayed to allow the other switch

to completely turn off.

In the Half-bridge Output mode, a digitally program-

mable

dead

band

delay

available

avoid

shoot-through current from destroying the bridge

power switches. The delay occurs at the signal transi-

tion from the non-active state to the active state. See

Figure 11-6 for illustration. The lower seven bits of the

PWM1CON register (Register 11-2) sets the delay

period in terms of microcontroller instruction cycles

(TCY or 4 TOSC).

11.3.7

ENHANCED PWM

AUTO-SHUTDOWN

When the ECCP is programmed for any of the

enhanced PWM modes, the active output pins may be

configured for auto-shutdown. Auto-shutdown immedi-

ately places the enhanced PWM output pins into a

defined shutdown state when a shutdown event

occurs.

A shutdown event can be caused by either of the two

comparators or the INT pin (or any combination of

these three sources). The comparators may be used to

monitor a voltage input proportional to a current being

monitored in the bridge circuit. If the voltage exceeds a

threshold, the comparator switches state and triggers a

shutdown. Alternatively, a digital signal on the INT pin

can also trigger a shutdown. The auto-shutdown

feature can be disabled by

not selecting any

auto-shutdown sources. The auto-shutdown sources to

be used are selected using the ECCPAS<2:0> bits

(ECCPAS<6:4>).

When a shutdown occurs, the output pins are

asynchronously placed in their shutdown states, spec-

ified by the PSSAC<1:0> and PSSBD<1:0> bits

(ECCPAS<3:0>).

Each

pair

(P1A/P1C

and

P1B/P1D) may be set to drive high, drive low, or be

tri-stated

(not

driving).

The

ECCPASE

bit

(ECCPAS<7>) is also set to hold the enhanced PWM

outputs in their shutdown states.

The ECCPASE bit is set by hardware when a shutdown

event occurs. If Auto-restarts are not enabled, the

ECCPASE bit is cleared by firmware when the cause of

the shutdown clears. If Auto-restarts are enabled, the

ECCPASE bit is automatically cleared when the cause

the

auto-shutdown

has

cleared.

See

Section 11.3.7.1 “Auto-shutdown and Auto-restart”

for more information.

PWM1CON – PWM CONFIGURATION REGISTER (ADDRESS: 16h)

R/W-0

PRSEN

PDC6

PDC5

PDC4

PDC3

PDC2

PDC1

PDC0

bit 7

bit 0

bit 7

PRSEN: PWM Restart Enable bit

= Upon auto-shutdown, the ECCPASE bit clears automatically once the shutdown event

goes away; the PWM restarts automatically.

= Upon auto-shutdown, ECCPASE must be cleared in software to restart the PWM.

bit 6-0

PDC<6:0>: PWM Delay Count bits

Number of FOSC/4 (4*TOSC) cycles between the scheduled time when a PWM signal should

transition active, and the actual time it transitions active.

Legend:

R = Readable bit

W = Writable bit

U = Unimplemented bit, read as ‘0’

-n = Value at POR

‘1’ = Bit is set

‘0’ = Bit is cleared

x = Bit is unknown

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