AT87F51
The AT87F51 provides the following standard features: 4K
bytes of QuickFlash, 128 bytes of RAM, 32 I/O lines, two
16-bit timer/counters, a five vector two-level interrupt archi-
tecture, a full duplex serial port, on-chip oscillator and clock
circuitry. In addition, the AT87F51 is designed with static
logic for operation down to zero frequency and supports
two software selectable power saving modes. The Idle
Mode stops the CPU while allowing the RAM,
timer/counters, serial port and interrupt system to continue
functioning. The Power Down Mode saves the RAM con-
tents but freezes the oscillator disabling all other chip func-
tions until the next hardware reset.
when emitting 1s. During accesses to external data mem-
ory 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 QuickFlash programming and verifi-
cation.
Port 3
Port 3 is an 8-bit bidirectional 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 internal pullups and can be used as inputs. As inputs,
Port 3 pins that are externally being pulled low will source
current (I
IL
) because of the pullups.
Port 3 also serves the functions of various special features
of the AT87F51 as listed below:
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)
Pin Description
V
CC
Supply voltage.
GND
Ground.
Port 0
Port 0 is an 8-bit open drain bidirectional I/O port. As an
output port each pin can sink eight TTL inputs. When 1s
are written to port 0 pins, the pins can be used as high-
impedance inputs.
Port 0 may also be configured to be the multiplexed low-
order address/data bus during accesses to external pro-
gram and data memory. In this mode P0 has internal pul-
lups.
Port 0 also receives the code bytes during QuickFlash pro-
gramming, and outputs the code bytes during program ver-
ification. External pullups are required during program veri-
fication.
Port 1
Port 1 is an 8-bit bidirectional I/O port with internal pullups.
The Port 1 output buffers can sink/source four TTL inputs.
When 1s are written to Port 1 pins they are pulled high by
the internal pullups and can be used as inputs. As inputs,
Port 1 pins that are externally being pulled low will source
current (I
IL
) because of the internal pullups.
Port 1 also receives the low-order address bytes during
QuickFlash programming and verification.
Port 2
Port 2 is an 8-bit bidirectional I/O port with internal pullups.
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 internal pullups and can be used as inputs. As inputs,
Port 2 pins that are externally being pulled low will source
current (I
IL
) because of the internal pullups.
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 pullups
Port 3 also receives some control signals for QuickFlash
programming and verification.
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 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 Quick-
Flash programming.
In normal operation ALE is emitted at a constant rate of 1/6
the oscillator frequency, and may be used for external tim-
ing or clocking purposes. Note, however, that one ALE
pulse is skipped during each access to external Data Mem-
ory.
If desired, ALE operation can be disabled by setting bit 0 of
SFR location 8EH. With the bit set, ALE is active only dur-
ing 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 pro-
gram memory.
3
ATMEL [ ATMEL CORPORATION ]
