AT89C51IC2_04 Datasheet, PDF (22/146 Pages) ATMEL Corporation – 8-bit Flash Microcontroller with 2-wire Interface

Figure 6. Mode Switching Waveforms
XTALA1
XTALA1:2
X2 bit
CPU clock
F_OSCA
STD Mode
X2 Mode
STD Mode
The X2 bit in the CKCON0 register (see Table 19) allow to switch from 12 clock periods
per instruction to 6 clock periods and vice versa. At reset, the speed is setting according
to X2 bit of Hardware Security Byte (HSB). By default, Standard mode is actived. Setting
the X2 bit activates the X2 feature (X2 mode).
The T0X2, T1X2, T2X2, UartX2, PcaX2, WdX2 and I2CX2 bits in the CKCON0 register
(See Table 19.) and SPIX2 bit in the CKCON1 register (see Table 20) allow to 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.
More information about the X2 mode can be found in the application note "How to take
advantage of the X2 features in TS80C51 microcontroller?"
22 AT89C51IC2
4301A–8051–01/04