T84C5112-RKFIL datasheet(8/97 Pages) ATMEL

8 / 97 page

Rev. B - November 10, 2000

Preliminary

T80C5112

6.2.3. Integrated low speed, low power RC oscillator : OSCC

The low speed, low power RC oscillator is used to clock the hardware watchdog and do not need any external

component; its typical frequency is 200 kHz.

This oscillator is also used as back up oscillator when the CPU receive no clock signal in active or idle modes.

RCLF_OFF is the configuration bit used to switch on or off the low speed RC oscillator.

Note that the on chip oscillator has a +-25% frequency tolerance and for that reason may not be suitable for use

in some applications.

6.2.4. Clock selector

CKS bit in CKS register is used to select from crystal to RC oscillator.

OSCBEN bit in OSCCON register is used to enable the RC oscillator.

OSCAEN bit in OSCCON register is used to enable the crystal oscillator or the external oscillator input.

If both oscillators are disabled, the low speed oscillator, OSCC is used as a backup source for the CPU and the

peripherals.

This feature provides a stand alone low frequency mode which can be activated when needed.

6.2.5. Clock prescaler

Before supplying the CPU and the peripherals, the main clock is divided by a factor to 2 to 512, as defined by

the CKRL register. The CPU needs from 12 to 256*12 clock periods per instruction. This allows:

To accept any cyclic ratio to be accepted on XTAL1 input.

To reduce the CPU power consumption.

The X2 bit allows to bypass the clock prescaler ; in this case, the CPU need only 6 clock periods per machine

cycle. In X2 mode, as this divider is bypassed, the signals on XTAL1 must have a cyclic ratio between 40 to 60%

T84C5112-RKFIL Datasheet(PDF) 8 Page - ATMEL Corporation