12C508 datasheet(38/113 Pages) MICROCHIP

38 / 113 page

PIC12C5XX

DS40139E-page 38

8.2.5

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 “Electri-

cal Specifications” section for information on variation

over voltage and temperature.

In addition, a calibration instruction is programmed into

the top of memory which contains the calibration value

for the internal RC oscillator. This location is never code

protected regardless of the code protect settings. This

value is programmed as a MOVLW XX instruction where

XX is the calibration value, and is placed at the reset

vector. This will load the W register with the calibration

value upon reset and the PC will then roll over to the

users program at address 0x000. The user then has the

option of writing the value to the OSCCAL Register

(05h) or ignoring it.

OSCCAL, when written to with the calibration value, will

“trim” the internal oscillator to remove process variation

from the oscillator frequency. .

For the PIC12C508A, PIC12C509A, PIC12CE518,

PIC12CE519, and PIC12CR509A, bits <7:2>, CAL5-

CAL0 are used for calibration. Adjusting CAL5-0 from

000000 to 111111 yields a higher clock speed. Note

that bits 1 and 0 of OSCCAL are unimplemented and

should be written as 0 when modifying OSCCAL for

compatibility with future devices.

For the PIC12C508 and PIC12C509, the upper 4 bits of

the register are used. Writing a larger value in this loca-

tion yields a higher clock speed.

8.3

RESET

The device differentiates between various kinds of

reset:

a) Power on reset (POR)

b) MCLR reset during normal operation

c) MCLR reset during SLEEP

d) WDT time-out reset during normal operation

e) WDT time-out reset during SLEEP

f) Wake-up from SLEEP on pin change

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 read prior to

erasing the part. so it can be repro-

grammed correctly later.

Some registers are not reset in any way; they are

unknown on POR and unchanged in any other reset.

Most other registers are reset to “reset state” on power-

on reset (POR), MCLR, WDT or wake-up on pin

change reset during normal operation. They are not

affected by a WDT reset during SLEEP or MCLR reset

during SLEEP, since these resets are viewed as

resumption of normal operation. The exceptions to this

are TO, PD, and GPWUF bits. They are set or cleared

differently in different reset situations. These bits are

used in software to determine the nature of reset. See

Table 8-3 for a full description of reset states of all

registers.

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