
14
AT89C51ID2
4289C–8051–11/05
Oscillators
Overview
Two oscillators are available (for AT8xC51IxD2 devices only, the others part number
provide only the main high frequency oscillator):
OSCA used for high frequency: Up to 40 MHz
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 16. CKSEL Register (for AT8xC51Ix2 only)
CKSEL - Clock Selection Register (85h)
Reset Value = 0000 000’HSB.OSC’b (see Hardware Security Byte (HSB))
Not bit addressable
76
543
210
--
----
-
CKS
Bit
Number
Bit
Mnemonic
Description
7-
Reserved
6-
Reserved
5-
Reserved
4-
Reserved
3-
Reserved
2-
Reserved
1-
Reserved
0CKS
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)