Rainbow Electronics T89C51RD2 User Manual
Page 11
11
Rev. F - 15 February, 2001
T89C51RD2
Figure 2. Mode Switching Waveforms
The X2 bit in the CKCON register (See Table 2.) allows to switch from 12 clock periods per instruction to 6
clock periods and vice versa. At reset, the standard speed is activated (STD mode). Setting this bit activates the
X2 feature (X2 mode).
The T0X2, T1X2, T2X2, SiX2, PcaX2 and WdX2 bits in the CKCON register (See Table 2.) 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 ANM072 "How to take advantage of
the X2 features in TS80C51 microcontroller?"
Table 2. CKCON Register
CKCON - Clock Control Register (8Fh)
7
6
5
4
3
2
1
0
-
WdX2
PcaX2
SiX2
T2X2
T1X2
T0X2
X2
Bit
Number
Bit
Mnemonic
Description
7
-
Reserved
6
WdX2
Watchdog clock (This control bit is validated when the CPU clock X2 is set; when X2 is low, this bit has no effect)
Clear to select 6 clock periods per peripheral clock cycle.
Set to select 12 clock periods per peripheral clock cycle.
5
PcaX2
Programmable Counter Array clock (This control bit is validated when the CPU clock X2 is set; when X2 is
low, this bit has no effect)
Clear to select 6 clock periods per peripheral clock cycle.
Set to select 12 clock periods per peripheral clock cycle.
4
SiX2
Enhanced UART clock (Mode 0 and 2) (This control bit is validated when the CPU clock X2 is set; when X2
is low, this bit has no effect)
Clear to select 6 clock periods per peripheral clock cycle.
Set to select 12 clock periods per peripheral clock cycle.
3
T2X2
Timer2 clock (This control bit is validated when the CPU clock X2 is set; when X2 is low, this bit has no effect)
Clear to select 6 clock periods per peripheral clock cycle.
Set to select 12 clock periods per peripheral clock cycle.
2
T1X2
Timer1 clock (This control bit is validated when the CPU clock X2 is set; when X2 is low, this bit has no effect)
Clear to select 6 clock periods per peripheral clock cycle.
Set to select 12 clock periods per peripheral clock cycle
XTAL1:2
XTAL1
CPU clock
X2 bit
X2 Mode
STD Mode
STD Mode