at89c51rd2-smsum ATMEL Corporation, at89c51rd2-smsum Datasheet - Page 18

• Manufacturer Part Number: at89c51rd2-smsum
• Description: 8-bit Flash Microcontroller
• Manufacturer: ATMEL Corporation

Figure 7-2.
18
AT89C51RD2/ED2
XTAL1
XTAL1:2
X2 Bit
CPU Clock
Mode Switching Waveforms
STD Mode
Figure 7-1.
The X2 bit in the CKCON0 register (see Table 7-1) allows a switch from 12 clock periods per
instruction to 6 clock periods and vice versa. At reset, the speed is set according to X2 bit of
Hardware Security Byte (HSB). By default, Standard mode is active. Setting the X2 bit activates
the X2 feature (X2 mode).
The T0X2, T1X2, T2X2, UartX2, PcaX2, and WdX2 bits in the CKCON0 register
SPIX2 bit in the CKCON1 register (see Table 7-2) allows a switch from standard peripheral
speed (12 clock periods per peripheral clock cycle) to fast peripheral speed (6 clock periods per
peripheral clock cycle). These bits are active only in X2 mode.
Table 7-1.
CKCON0 - Clock Control Register (8Fh)
Number
Bit
7
7
6
-
XTAL1
Mnemonic
Reserved
Clock Generation Diagram
CKCON0 Register
WDX2
WDX2
Bit
6
FXTAL
Description
The values for this bit are indeterminite. Do not set this bit.
Watchdog Clock
(This control bit is validated when the CPU clock X2 is set; when X2 is low, this bit has no
effect).
Cleared to select 6 clock periods per peripheral clock cycle.
Set to select 12 clock periods per peripheral clock cycle.
PCAX2
5
2
F
XTAL1:2
X2 Mode
OSC
SIX2
4
CKCON0
X2
0
1
F
OSC
T2X2
3
8-bit Prescaler
CKRL
T1X2
2
STD Mode
T0X2
1
(Table
F
F
4235J–8051–01/08
CLK CPU
CLK PERIPH
7-1) and
X2
0