PIC16F84A datasheet(23/68 Pages) MICROCHIP

23 / 68 page

PIC16F84A

Preliminary

DS35007A-page 23

6.2.3

RC OSCILLATOR

For timing insensitive applications the RC device option

offers additional cost savings. The RC oscillator

frequency is a function of the supply voltage, the

resistor (Rext) values, capacitor (Cext) values, and the

operating temperature. In addition to this, the oscillator

frequency will vary from unit to unit due to normal

process

parameter

variation.

Furthermore,

the

difference in lead frame capacitance between package

types also affects the oscillation frequency, especially

for low Cext values. The user needs to take into

account variation due to tolerance of the external

R and C components. Figure 6-4 shows how an R/C

combination is connected to the PIC16F84A.

FIGURE 6-4:

RC OSCILLATOR MODE

6.3

Reset

The PIC16F84A 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)

Figure 6-5 shows a simpliﬁed block diagram of the

on-chip reset circuit. The MCLR reset path has a noise

ﬁlter to ignore small pulses. The electrical speciﬁca-

tions state the pulse width requirements for the MCLR

pin.

Some registers are not affected in any reset condition;

their status is unknown on a POR reset and unchanged

in any other reset. Most other registers are reset to a

“reset state” on POR, MCLR or WDT reset during

normal operation and on MCLR reset during SLEEP.

They are not affected by a WDT reset during SLEEP,

since this reset is viewed as the resumption of normal

operation.

Table 6-3 gives a description of reset conditions for the

program counter (PC) and the STATUS register.

Table 6-4 gives a full description of reset states for all

registers.

The TO and PD bits are set or cleared differently in dif-

ferent reset situations (Section 6.7). These bits are

used in software to determine the nature of the reset.

FIGURE 6-5:

SIMPLIFIED BLOCK DIAGRAM OF ON-CHIP RESET CIRCUIT

OSC2/CLKOUT

Cext

Rext

PIC16FXX

OSC1

Fosc/4

Internal

clock

VDD

VSS

Recommended values:

5 k

Ω ≤ Rext ≤ 100 kΩ

Cext > 20pF

External

Reset

MCLR

VDD

OSC1/

WDT

Module

VDD rise

detect

OST/PWRT

On-chip

RC OSC(1)

WDT

Time_Out

Power_on_Reset

OST

10-bit Ripple counter

PWRT

Chip_Reset

10-bit Ripple counter

Reset

Enable OST

Enable PWRT

SLEEP

CLKIN

Note 1:

This is a separate oscillator from the

RC oscillator of the CLKIN pin.

See Table 6-5

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