PIC16F872 Datasheet, PDF (99/160 Pages) Microchip Technology – 28-Pin, 8-Bit CMOS FLASH Microcontroller

PIC16F872
11.3 Reset
The PIC16F872 differentiates between various kinds of
reset:
• Power-on Reset (POR)
• MCLR Reset during normal operation
• MCLR Reset during SLEEP
• WDT Reset (during normal operation)
• WDT Wake-up (during SLEEP)
• Brown-out Reset (BOR)
Some registers are not affected in any RESET condi-
tion. Their status is unknown on POR and unchanged
in any other RESET. Most other registers are reset to a
“reset state” on Power-on Reset (POR), on the MCLR
and WDT Reset, on MCLR Reset during SLEEP, and
Brown-out Reset (BOR). They are not affected by a
WDT Wake-up, which is viewed as the resumption of
normal operation. The TO and PD bits are set or
cleared differently in different reset situations as indi-
cated in Table 11-4. These bits are used in software to
determine the nature of the reset. See Table 11-6 for a
full description of reset states of all registers.
A simplified block diagram of the on-chip reset circuit is
shown in Figure 11-4.
These devices have a MCLR noise filter in the MCLR
Reset path. The filter will detect and ignore small
pulses.
It should be noted that a WDT Reset does not drive
MCLR pin low.
FIGURE 11-4: SIMPLIFIED BLOCK DIAGRAM OF ON-CHIP RESET CIRCUIT
External
Reset
MCLR
VDD
OSC1
WDT
Module
VDD rise
detect
SLEEP
WDT
Time-out
Reset
Power-on Reset
Brown-out
Reset
BODEN
OST/PWRT
OST
10-bit Ripple counter
(1)
On-chip
RC OSC
PWRT
10-bit Ripple counter
S
Chip_Reset
R
Q
Enable PWRT
Enable OST
Note 1: This is a separate oscillator from the RC oscillator of the CLKIN pin.
© 1999 Microchip Technology Inc.
Preliminary
DS30221A-page 99