at89c51re2-slsem ATMEL Corporation, at89c51re2-slsem Datasheet - Page 14

• Manufacturer Part Number: at89c51re2-slsem
• Description: At89c51re2 8-bit Flash Microcontroller
• Manufacturer: ATMEL Corporation

Enhanced Features
X2 Feature
Description
14
AT89C51RE2
In comparison to the original 80C52, the AT89C51RE2 implements some new features,
which are
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The AT89C51RE2 core needs only 6 clock periods per machine cycle. This feature
called ‘X2’ provides the following advantages:
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In order to keep the original C51 compatibility, a divider by 2 is inserted between the
XTAL1 signal and the main clock input of the core (phase generator). This divider may
be disabled by software.
The clock for the whole circuit and peripherals is first divided by two before being used
by the CPU core and the peripherals.
This allows any cyclic ratio to be accepted on XTAL1 input. In X2 mode, as this divider is
bypassed, the signals on XTAL1 must have a cyclic ratio between 40 to 60%.
Figure 2 shows the clock generation block diagram. X2 bit is validated on the rising edge
of the XTAL1÷2 to avoid glitches when switching from X2 to STD mode. Figure 3 shows
the switching mode waveforms.
Figure 2. Clock Generation Diagram
XTAL1
X2 option
Dual Data Pointer
Extended RAM
Extended stack
Programmable Counter Array (PCA)
Hardware Watchdog
SPI interface
4-level interrupt priority system
power-off flag
ONCE mode
ALE disabling
Enhanced features on the UART and the timer 2
Divide frequency crystals by 2 (cheaper crystals) while keeping same CPU power.
Save power consumption while keeping same CPU power (oscillator power saving).
Save power consumption by dividing dynamically the operating frequency by 2 in
operating and idle modes.
Increase CPU power by 2 while keeping same crystal frequency.
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FXTAL
2
XTAL1:2
CKCON0
X2
0
1
F
OSC
8 bit Prescaler
CKRL
7663B–8051–03/07
F
F
CLK CPU
CLK PERIPH