AT89C51ID2-SLSIM Atmel, AT89C51ID2-SLSIM Datasheet - Page 17

• Manufacturer Part Number: AT89C51ID2-SLSIM
• Description: C51ID2 64KF TWI 32KHZ PLCC44 3-5.5V IND.
• Manufacturer: Atmel

Functional Block
Diagram
Figure 2. Functional Oscillator Block Diagram
Operating Modes
Reset
Functional Modes
Normal Modes
17
XtalA1
XtalA2
AT89C51ID2
PwdOscA
XtalB1
XtalB2
OSCCON
OscAEn
OscA
PwdOscB
FOSCA
OSCCON
OscB
OscBEn
:2
Reset
A hardware RESET puts the Clock generator in the following state:
The selected oscillator depends on OSC bit in Hardware Security Byte (HSB).
HSB.OSC = 1 (Oscillator A selected)
•
•
HSB.OSC = 0 (Oscillator B selected)
•
•
•
•
•
•
OscAEn = 1 & OscBEn = 0: OscA is running, OscB is stopped.
CKS = 1: OscA is selected for CPU.
OscAEn = 0 & OscBEn = 1: OscB is running, OscA is stopped.
CKS = 0: OscB is selected for CPU.
CPU and Peripherals clock depend on the software selection using CKCON0,
CKCON1 and CKRL registers
CKS bit in CKSEL register selects either OscA or OscB
CKRL register determines the frequency of the OscA clock.
It is always possible to switch dynamically by software from OscA to OscB, and vice
versa by changing CKS bit.
CKCON0
CKRL
X2
0
1
Prescaler-Divider
8-bit
Reload
CKRL=0xFF?
FOSCB
0
1
CKSEL
CKS
1
0
:128
Idle
CLK
PERIPH
CLK
CPU
Sub
Clock
Peripheral Clock
4289C–8051–11/05
CPU clock