PIC12C67X datasheet(56/129 Pages) MICROCHIP

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PIC12C67X

DS30561B-page 56

9.2.5

INTERNAL 4 MHz RC OSCILLATOR

The internal RC oscillator provides a fixed 4 MHz (nom-

inal) system clock at VDD = 5V and 25°C. See

Section 13.0 for information on variation over voltage

and temperature.

In addition, a calibration instruction is programmed into

the last address of the program memory which contains

the calibration value for the internal RC oscillator. This

value is programmed as a RETLW XX instruction where

XX is the calibration value. In order to retrieve the cali-

bration value, issue a CALL YY instruction where YY is

the last location in program memory (03FFh for the

PIC12C671 and the PIC12CE673, 07FFh for the

PIC12C672 and the PIC12CE674). Control will be

returned to the user’s program with the calibration

value loaded into the W register. The program should

then perform a MOVWF OSCCAL instruction to load the

value into the internal RC oscillator trim register.

OSCCAL, when written to with the calibration value, will

“trim” the internal oscillator to remove process variation

from the oscillator frequency. Bits <7:4>, CAL<3:0> are

used for fine calibration, while bit 3, CALFST, and bit 2,

CALSLW, are used for more coarse adjustment. Adjust-

ing CAL<3:0> from 0000 to 1111 yields a higher clock

speed. Set CALFST = 1 for greater increase in fre-

quency or set CALSLW = 1 for greater decrease in fre-

quency. Note that bits 1 and 0 of OSCCAL are

unimplemented and should be written as 0 when mod-

ifying OSCCAL for compatibility with future devices.

9.2.6

CLKOUT

The PIC12C67X can be configured to provide a clock

out signal (CLKOUT) on pin 3 when the configuration

word address (2007h) is programmed with FOSC2,

FOSC1, and FOSC0, equal to 101 for INTRC or 111 for

EXTRC. The oscillator frequency, divided by 4, can be

used for test purposes or to synchronize other logic.

Note:

Please note that erasing the device will

also erase the pre-programmed internal

calibration value for the internal oscillator.

The calibration value must be saved prior

to erasing the part.

9.3

Reset

The PIC12C67X differentiates between various kinds

of reset:

• Power-on Reset (POR)

•MCLR Reset during normal operation

•MCLR Reset during SLEEP

• WDT Reset (normal operation)

Some registers are not affected in any reset condition;

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), MCLR Reset, WDT

Reset, and MCLR Reset during SLEEP. 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 indicated in Table 9-5. These bits are used in

software to determine the nature of the reset. See

Table 9-6 for a full description of reset states of all

registers.

A simplified block diagram of the on-chip reset circuit is

shown in Figure 9-6.

The PIC12C67X has 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.

When MCLR is asserted, the state of the OSC1/CLKIN

and CLKOUT/OSC2 pins are as follows:

TABLE 9-3:

CLKIN/CLKOUT PIN STATES

WHEN MCLR ASSERTED

Oscillator Mode

OSC1/CLKIN Pin

OSC2/CLKout Pin

EXTRC, CLKOUT

on OSC2

OSC1 pin is

tristated and

driven by external

circuit

OSC2 pin is driven

low

EXTRC, OSC2 is

I/O

OSC1 pin is

tristated and

driven by external

circuit

OSC2 pin is

tristate input

INTRC, CLKOUT

on OSC2

OSC1 pin is

tristate input

OSC2 pin is driven

low

INTRC, OSC2 is

I/O

OSC1 pin is

tristate input

OSC2 pin is

tristate input

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