P12C508 datasheet(28/84 Pages) MICROCHIP

28 / 84 page

PIC12C5XX

DS40139A-page 28

Advance Information

FIGURE 7-6:

RC OSCILLATOR MODE

7.2.5

INTERNAL 4 MHZ RC OSCILLATOR

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

inal) system clock.

In addition, a calibration instruction is programmed into

the top of memory which indicates the calibration value

for the internal RC oscillator. This value, OSCCAL, 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 0x000. The user

then has the option of writing the value to the OSCCAL

VDD

Rext

Cext

VSS

OSC1

Internal

clock

PIC12C5XX

7.3

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

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), on MCLR or WDT 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 sit-

uations. These bits are used in software to determine

the nature of reset. See Table 7-3 for a full description

of reset states of all registers.

TABLE 7-3:

RESET CONDITIONS FOR REGISTERS

Address

Power-on Reset

MCLR Reset

WDT time-out

Wake-up on Pin Change

W—

qqqq xxxx (1)

qqqq uuuu (1)

INDF

00h

xxxx xxxx

uuuu uuuu

TMR0

01h

xxxx xxxx

uuuu uuuu

02h

1111 1111

STATUS

03h

0001 1xxx

?00? ?uuu (2)

FSR (12C508)

04h

111x xxxx

111u uuuu

FSR (12C509)

04h

110x xxxx

11uu uuuu

OSCCAL

05h

0111 ----

uuuu ----

GPIO

06h

--xx xxxx

--uu uuuu

OPTION

—

1111 1111

TRIS

—

--11 1111

Legend: u = unchanged, x = unknown, - = unimplemented bit, read as ‘0’, ? = value depends on condition.

Note 1:

Bits <7:4> of W register contain oscillator calibration (OSCCAL) values due to MOVLW XX instruction at

top of memory.

Note 2:

See Table 7-6 for reset value for speciﬁc conditions

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