AT89S8252
Port Pin
P3.0
P3.1
P3.2
P3.3
P3.4
P3.5
P3.6
P3.7
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)
RST
Reset input. A high on this pin for two machine cycles while the oscillator is running
resets the device.
ALE/PROG
Address Latch Enable 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.
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 during each access to external data memory.
If desired, ALE operation can be disabled by setting bit 0 of SFR 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 execution mode.
PSEN
Program Store Enable is the read strobe to external program memory.
When the AT89S8252 is executing code from external program memory, PSEN is acti-
vated twice each machine cycle, except that two PSEN activations are skipped during
each access to external data memory.
EA/VPP
External Access Enable. EA must be strapped to GND in order to enable the device to
fetch code from external program memory locations starting at 0000H up to FFFFH.
Note, however, that if lock bit 1 is programmed, EA will be internally latched on reset.
EA should be strapped to VCC for internal program executions. This pin also receives the
12-volt programming enable voltage (VPP) during Flash programming when 12-volt pro-
gramming is selected.
XTAL1
XTAL2
Input to the inverting oscillator amplifier and input to the internal clock operating circuit.
Output from the inverting oscillator amplifier.
5
0401G–MICRO–3/06
ATMEL [ ATMEL ]
