PIC16CR54C-20I/SO View Datasheet(PDF) - Microchip Technology

Part Name

Description

Manufacturer

PIC16CR54C-20I/SO

EPROM/ROM-Based 8-bit CMOS Microcontroller Series

Microchip Technology 

PIC16C5X

8.2 Prescaler

An 8-bit counter is available as a prescaler for the

Timer0 module, or as a postscaler for the Watchdog

Timer (WDT), respectively (Section 9.2.1). For simplic-

ity, this counter is being referred to as “prescaler”

throughout this data sheet. Note that the prescaler may

be used by either the Timer0 module or the WDT, but

not both. Thus, a prescaler assignment for the Timer0

module means that there is no prescaler for the WDT,

and vice-versa.

The PSA and PS<2:0> bits (OPTION<3:0>) determine

prescaler assignment and prescale ratio.

When assigned to the Timer0 module, all instructions

writing to the TMR0 register (e.g., CLRF 1,

MOVWF 1, BSF 1,x, etc.) will clear the prescaler.

When assigned to WDT, a CLRWDT instruction will clear

the prescaler along with the WDT. The prescaler is nei-

ther readable nor writable. On a RESET, the prescaler

contains all '0's.

8.2.1

SWITCHING PRESCALER

ASSIGNMENT

The prescaler assignment is fully under software con-

trol (i.e., it can be changed “on the fly” during program

execution). To avoid an unintended device RESET, the

following instruction sequence (Example 8-1) must be

executed when changing the prescaler assignment

from Timer0 to the WDT.

EXAMPLE 8-1:

CHANGING PRESCALER

(TIMER0WDT)

CLRWDT

CLRF TMR0

MOVLW B'00xx1111’

OPTION

CLRWDT

MOVLW B'00xx1xxx’

OPTION

;Clear WDT

;Clear TMR0 & Prescaler

;Last 3 instructions in

this example

;are required only if

;desired

;PS<2:0> are 000 or

;001

;Set Prescaler to

;desired WDT rate

To change prescaler from the WDT to the Timer0 mod-

ule, use the sequence shown in Example 8-2. This

sequence must be used even if the WDT is disabled. A

CLRWDT instruction should be executed before switch-

ing the prescaler.

EXAMPLE 8-2:

CHANGING PRESCALER

(WDTTIMER0)

CLRWDT

MOVLW B'xxxx0xxx'

OPTION

;Clear WDT and

;prescaler

;Select TMR0, new

;prescale value and

;clock source

DS30453E-page 40

Preliminary

 1997-2013 Microchip Technology Inc.

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