at89s51-33sc ATMEL Corporation, at89s51-33sc Datasheet - Page 5

• Manufacturer Part Number: at89s51-33sc
• Description: At89s51, 8-bit Microcontroller With 4k Bytes In-system Programmable Flash
• Manufacturer: ATMEL Corporation

4.8
4.9
4.10
4.11
2487C–MICRO–03/05
Port 2
Port 3
RST
ALE/PROG
Port 2 is an 8-bit bi-directional I/O port with internal pull-ups. The Port 2 output buffers can
sink/source four TTL inputs. When 1s are written to Port 2 pins, they are pulled high by the inter-
nal pull-ups and can be used as inputs. As inputs, Port 2 pins that are externally being pulled low
will source current (I
Port 2 emits the high-order address byte during fetches from external program memory and dur-
ing accesses to external data memory that use 16-bit addresses (MOVX @ DPTR). In this
application, Port 2 uses strong internal pull-ups when emitting 1s. During accesses to external
data memory that use 8-bit addresses (MOVX @ RI), Port 2 emits the contents of the P2 Special
Function Register.
Port 2 also receives the high-order address bits and some control signals during Flash program-
ming and verification.
Port 3 is an 8-bit bi-directional I/O port with internal pull-ups. The Port 3 output buffers can
sink/source four TTL inputs. When 1s are written to Port 3 pins, they are pulled high by the inter-
nal pull-ups and can be used as inputs. As inputs, Port 3 pins that are externally being pulled low
will source current (I
Port 3 receives some control signals for Flash programming and verification.
Port 3 also serves the functions of various special features of the AT89S51, as shown in the fol-
lowing table.
Reset input. A high on this pin for two machine cycles while the oscillator is running resets
the device. This pin drives High for 98 oscillator periods after the Watchdog times out. The DIS-
RTO bit in SFR AUXR (address 8EH) can be used to disable this feature. In the default state of
bit DISRTO, the RESET HIGH out feature is enabled.
Address Latch Enable (ALE) is an output pulse for latching the low byte of the address during
accesses to external memory. This pin is also the program pulse input (PROG) during Flash
programming.
Port Pin
P3.0
P3.1
P3.2
P3.3
P3.4
P3.5
P3.6
P3.7
IL
IL
) because of the internal pull-ups.
) because of the pull-ups.
Alternate Functions
RXD (serial input port)
TXD (serial output port)
INT0 (external interrupt 0)
INT1 (external interrupt 1)
T0 (timer 0 external input)
T1 (timer 1 external input)
WR (external data memory write strobe)
RD (external data memory read strobe)
AT89S51
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