TS80C51RA2 Atmel Corporation, TS80C51RA2 Datasheet - Page 12

• Manufacturer Part Number: TS80C51RA2
• Description: High Performance 8-bit Microcontroller
• Manufacturer: Atmel Corporation

Figure 2. Mode Switching Waveforms
12
TS8xC51Rx2
XTAL1
XTAL1:2
X2 bit
CPU clock
STD Mode
The X2 bit in the CKCON register (Table 3) allows to switch from 12 clock cycles per
instruction to 6 clock cycles and vice versa. At reset, the standard speed is activated
(STD mode). Setting this bit activates the X2 feature (X2 mode).
Note:
Table 3. CKCON Register
CKCON - Clock Control Register (8Fh)
Reset Value = XXXX XXX0b
Not bit addressable
For further details on the X2 feature, please refer to ANM072 available on the web
(http://www.atmel.com)
Bit Number
7
-
7
6
5
4
3
2
1
0
In order to prevent any incorrect operation while operating in X2 mode, user must be
aware that all peripherals using clock frequency as time reference (UART, timers, PCA...)
will have their time reference divided by two. For example a free running timer generating
an interrupt every 20 ms will then generate an interrupt every 10 ms. UART with 4800
baud rate will have 9600 baud rate.
Mnemonic
6
Bit
X2
-
-
-
-
-
-
-
-
Description
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
Reserved
The value read from this bit is indeterminate. Do not set this bit.
CPU and peripheral clock bit
Clear to select 12 clock periods per machine cycle (STD mode, F
Set to select 6 clock periods per machine cycle (X2 mode, F
X2 Mode
5
-
4
-
3
-
2
-
STD Mode
1
-
OSC
4188A–8051–10/02
=F
OSC
XTAL
=F
).
XTAL
X2
0
/
2).