
6
7663E–8051–10/08
AT89C51RE2
15
9
I
INT1 (P3.3): External interrupt 1
16
10
I
T0 (P3.4): Timer 0 external input
17
11
I
T1 (P3.5): Timer 1 external input
18
12
O
WR (P3.6): External data memory write strobe
19
13
O
RD (P3.7): External data memory read strobe
P6.0-P6.1
12,34
6, 28
Port 6: Port 6 is an 2-bit bidirectional I/O port with internal pull-ups. Port 6 pins that have 1s
written to them are pulled high by the internal pull-ups and can be used as inputs. As inputs,
Port 6 pins that are externally pulled low will source current because of the internal pull-ups.
Port 6 also serves some special features as listed below.
12
6
I
RXD_1 (P6.0): Serial input port
12
6
I/O
SDA (P6.0) : TWI Serial Data
SDA is the bidirectional TWI data line.
34
28
O
TXD_1 (P6.1) : Serial output port
34
28
I/O
SCL ( P6.1) : TWI Serial Clock
SCL output the serial clock to slave peripherals.
SCL input the serial clock from master.
Reset
10
4
I/O
Reset: A high on this pin for two machine cycles while the oscillator is running, resets the
device. An internal diffused resistor to V
SS permits a power-on reset using only an external
capacitor to VCC. This pin is an output when the hardware watchdog forces a system reset.
ALE/PROG
33
27
O (I)
Address Latch Enable/Program Pulse: Output pulse for latching the low byte of the
address during an access to external memory. In normal operation, ALE is emitted at a
constant rate of 1/6 (1/3 in X2 mode) the oscillator frequency, and can be used for external
timing or clocking. Note that one ALE pulse is skipped during each access to external data
memory. This pin is also the program pulse input (PROG) during Flash programming. ALE
can be disabled by setting SFR’s AUXR.0 bit. With this bit set, ALE will be inactive during
internal fetches.
PSEN
32
26
O
Program Store ENable: The read strobe to external program memory. When executing
code from the external program memory, PSEN is activated twice each machine cycle,
except that two PSEN activations are skipped during each access to external data memory.
PSEN is not activated during fetches from internal program memory.
EA
35
29
I
External Access Enable: EA must be externally held low to enable the device to fetch code
from external program memory locations 0000H to FFFFH (RD). If security level 1 is
programmed, EA will be internally latched on Reset.
XTAL1
21
15
I
Crystal 1: Input to the inverting oscillator amplifier and input to the internal clock generator
circuits.
XTAL2
20
14
O
Crystal 2: Output from the inverting oscillator amplifier
Tx_OCD
23
17
O
Tx_OCD: On chip debug Serial output port
Rx_OCD
1
39
I
Rx_OCD: On chip debug Serial input port
Mnemonic
Pin Number
Type
Name and Function
LCC
VQFP 1.4