5
7663E–8051–10/08
AT89C51RE2
Table 2. Pin Description
Mnemonic
Pin Number
Type
Name and Function
LCC
VQFP 1.4
V
SS
22
16
I
Ground: 0V reference
Vss1
39
I
Optional Ground: Contact the Sales Office for ground connection.
V
CC
44
38
I
Power Supply: This is the power supply voltage for normal, idle and power-down operation
P0.0-P0.7
43-36
37-30
I/O
Port 0: Port 0 is an open-drain, bidirectional I/O port. Port 0 pins that have 1s written to them
float and can be used as high impedance inputs. Port 0 must be polarized to VCC or VSS in
order to prevent any parasitic current consumption. Port 0 is also the multiplexed low-order
address and data bus during access to external program and data memory. In this
application, it uses strong internal pull-up when emitting 1s. Port 0 also inputs the code bytes
during EPROM programming. External pull-ups are required during program verification
during which P0 outputs the code bytes.
P1.0-P1.7
2-9
40-44
1-3
I/O
Port 1: Port 1 is an 8-bit bidirectional I/O port with internal pull-ups. Port 1 pins that have 1s
written to them are pulled high by the internal pull-ups and can be used as inputs. As inputs,
Port 1 pins that are externally pulled low will source current because of the internal pull-ups.
Port 1 also receives the low-order address byte during memory programming and
verification.
Alternate functions for TSC8x54/58 Port 1 include:
240
I/O
T2 (P1.0): Timer/Counter 2 external count input/Clockout
341
I
T2EX (P1.1): Timer/Counter 2 Reload/Capture/Direction Control
442
I
ECI (P1.2): External Clock for the PCA
543
I/O
CEX0 (P1.3): Capture/Compare External I/O for PCA module 0
644
I/O
CEX1 (P1.4): Capture/Compare External I/O for PCA module 1
71
I/O
CEX2 (P1.5): Capture/Compare External I/O for PCA module 2
82
I/O
CEX3 (P1.6): Capture/Compare External I/O for PCA module 3
93
I/O
CEX4 (P1.7): Capture/Compare External I/O for PCA module 4
P2.0-P2.7
24-31
18-25
I/O
Port 2: Port 2 is an 8-bit bidirectional I/O port with internal pull-ups. Port 2 pins that have 1s
written to them are pulled high by the internal pull-ups and can be used as inputs. As inputs,
Port 2 pins that are externally pulled low will source current because of the internal pull-ups.
Port 2 emits the high-order address byte during fetches from external program memory and
during accesses to external data memory that use 16-bit addresses (MOVX @DPTR).In this
application, it uses strong internal pull-ups emitting 1s. During accesses to external data
memory that use 8-bit addresses (MOVX @Ri), port 2 emits the contents of the P2 SFR.
Some Port 2 pins receive the high order address bits during EPROM programming and
verification:
P2.0 to P2.5 for RB devices
P2.0 to P2.6 for RC devices
P2.0 to P2.7 for RD devices.
P3.0-P3.7
11,
13-19
5,
7-13
I/O
Port 3: Port 3 is an 8-bit bidirectional I/O port with internal pull-ups. Port 3 pins that have 1s
written to them are pulled high by the internal pull-ups and can be used as inputs. As inputs,
Port 3 pins that are externally pulled low will source current because of the internal pull-ups.
Port 3 also serves the special features of the 80C51 family, as listed below.
11
5
I
RXD_0 (P3.0): Serial input port
13
7
O
TXD_0 (P3.1): Serial output port
14
8
I
INT0 (P3.2): External interrupt 0