at89lv55 ATMEL Corporation, at89lv55 Datasheet - Page 5

• Manufacturer Part Number: at89lv55
• Description: At89lv55 8-bit Microcontroller With 20k Bytes Flash
• Manufacturer: ATMEL Corporation

4.6
4.7
4.8
4.9
0811D–MICRO–3/06
Port 3
RST
ALE/PROG
PSEN
Port 3 is an 8-bit bi-directional I/O port with internal pullups. 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 pullups and can be used as inputs. As inputs, Port 3 pins that are externally being pulled low
will source current (I
Port 3 also serves the functions of various special features of the AT89LV55, as shown in the
following table.
Port 3 also receives the highest-order address bit and some control signals for Flash program-
ming and verification.
Reset input. A high on this pin for two machine cycles while the oscillator is running resets the
device.
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 dur-
ing 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.
Program Store Enable is the read strobe to external program memory.
When the AT89LV55 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 exter-
nal data memory.
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)
IL
) because of the pullups.
AT89LV55
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