at89c55-12qi ATMEL Corporation, at89c55-12qi Datasheet - Page 3

• Manufacturer Part Number: at89c55-12qi
• Description: 8-bit Microcontroller With Bytes Flash
• Manufacturer: ATMEL Corporation

Description
The AT89C55 provides the following standard features:
20K bytes of Flash, 256-bytes of RAM, 32 I/O lines, three
16 bit timer/counters, a six-vector two-level interrupt archi-
tecture, a full duplex serial port, on-chip oscillator, and
clock circuitry. In addition, the AT89C55 is designed with
static logic for operation down to zero frequency and sup-
ports two software selectable power saving modes. The
Idle Mode stops the CPU while allowing the RAM,
timer/counters, serial port, and interrupt system to con-
tinue functioning. The Power Down Mode saves the RAM
contents but freezes the oscillator, disabling all other chip
functions until the next hardware reset. The low voltage
option saves power and operates with a 2.7-volt power
supply.
Pin Description
V
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-im-
pedance inputs.
Port 0 can also be configured to be the multiplexed low-or-
der 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 Flash program-
ming and outputs the code bytes during program verifica-
tion. External pullups are required during program verifica-
tion.
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
In addition, P1.0 and P1.1 can be configured to be the
timer/counter 2 external count input (P1.0/T2) and the
timer/counter 2 trigger input (P1.1/T2EX), respectively, as
shown in the following table.
CC
IL
) because of the internal pullups.
(Continued)
Port 1 also receives the low-order address bytes during
Flash programming and program 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
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, Port 2 uses strong internal
pullups 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 programming and verification.
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
Port 3 also serves the functions of various special features
of the AT89C55, as shown in the following table.
Port 3 also receives some control signals for Flash pro-
gramming and programming verification.
RST
Reset input. A high on this pin for two machine cycles
while the oscillator is running resets the device.
Port Pin
P1.0
P1.1
Port Pin
P3.0
P3.1
P3.2
P3.3
P3.4
P3.5
P3.6
P3.7
IL
IL
) because of the internal pullups.
) because of the pullups.
Alternate Functions
T2 (external count input to
Timer/Counter 2), clock-out
T2EX (Timer/Counter 2 capture/reload
trigger and direction control)
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)
AT89C55
(continued)
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