AT89S8253
3.9
Port 3
Port 3 is an 8-bit bi-directional I/O port with internal pull-ups. The Port 3 output buffers can
sink/source six TTL inputs. When 1s are written to Port 3 pins, they are pulled high by the weak
internal pull-ups and can be used as inputs. As inputs, Port 3 pins that are externally being
pulled low will source current (I
IL
,150
µ
A typical) because of the weak internal pull-ups.
Port 3 receives some control signals for Flash programming and verification.
Port 3 also serves the functions of various special features of the AT89S8253, as shown in the
following table.
Port Pin
P3.0
P3.1
P3.2
P3.3
P3.4
P3.5
P3.6
P3.7
Note:
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)
1. All pins in ports 1 and 2 and almost all pins in port 3 (the exceptions are P3.2 INT0 and P3.3
INT1) have their inputs disabled in the Power-down mode. Port pins P3.2 (INT0) and P3.3
(INT1) are active even in Power-down mode (to be able to sense an interrupt request to exit
the Power-down mode) and as such still have their weak internal pull-ups turned on.
3.10
RST
Reset input. A high on this pin for at least two machine cycles while the oscillator is running
resets the device.
3.11
ALE/PROG
Address Latch Enable. ALE/PROG is an output pulse for latching the low byte of the address (on
its falling edge) during accesses to external memory. This pin is also the program pulse input
(PROG) during Flash programming.
In normal operation, ALE is emitted at a constant rate of 1/6 the oscillator frequency and may be
used for external timing or clocking purposes. Note, however, that one ALE pulse is skipped dur-
ing each access to external data memory.
If desired, ALE operation can be disabled by setting bit 0 of the AUXR SFR at location 8EH. With
the bit set, ALE is active only during a MOVX or MOVC instruction. Otherwise, the pin is weakly
pulled high. Setting the ALE-disable bit has no effect if the microcontroller is in external execu-
tion mode.
3.12
PSEN
Program Store Enable. PSEN is the read strobe to external program memory (active low).
When the AT89S8253 is executing code from external program memory, PSEN is activated
twice each machine cycle, except that two PSEN activations are skipped during each access to
external data memory.
5
3286M–MICRO–10/09
ATMEL [ ATMEL CORPORATION ]
