Datasheet

AT89C51IC2

4301D–8051–02/08

Oscillators

Overview Two oscillators are available for CPU:

• OSCA used for high frequency: Up to 48 MHz @5V +/- 10%

• OSCB used for low frequency: 32.768 kHz

Several operating modes are available and programmable by software:

• to switch OSCA to OSCB and vice-versa

• to stop OSCA or OSCB to reduce consumption

In order to optimize the power consumption and the execution time needed for a specific

task, an internal prescaler feature has been implemented between the selected oscilla-

tor and the CPU.

Registers Table 14. CKSEL Register

CKSEL - Clock Selection Register (85h)

Reset Value = 0000 000’HSB.OSC’b (see Hardware Security Byte (HSB) Table 84)

Not bit addressable

7 6 5 4 3 2 1 0

- - - - - - - CKS

Bit

Number

Bit

Mnemonic Description

7 - Reserved

6 - Reserved

5 - Reserved

4 - Reserved

3 - Reserved

2 - Reserved

1 - Reserved

0 CKS

CPU Oscillator Select Bit: (CKS)

Cleared, CPU and peripherals connected to OSCB

Set, CPU and peripherals connected to OSCA

Programmed by hardware after a Power-up regarding Hardware Security Byte

(HSB).HSB.OSC (Default setting, OSCA selected)