AT89LP51 Atmel Corporation, AT89LP51 Datasheet - Page 42

• Manufacturer Part Number: AT89LP51
• Manufacturer: Atmel Corporation

Specifications of AT89LP51

• Flash (kbytes): 4 Kbytes
• Max. Operating Frequency: 20 MHz
• Cpu: 8051-1C
• Max I/o Pins: 36
• Uart: 1
• Sram (kbytes): 0.25
• Eeprom (bytes): 256
• Self Program Memory: IAP
• Operating Voltage (vcc): 2.4 to 5.5
• Timers: 3
• Isp: SPI
• Watchdog: Yes

Table 10-3.
10.1.1
10.1.2
42
Symbol
P3M
P2M
P1M
P0M
PMOD = C1H
Not Bit Addressable
Bit
1-0
1-0
1-0
1-0
AT89LP51/52
Quasi-bidirectional Output
Input-only Mode
Function
Port 3 Configuration Mode
Port 2 Configuration Mode
Port 1 Configuration Mode
Port 0 Configuration Mode
PMOD
P3M1
7
– Port Mode Register
.
Port pins in quasi-bidirectional output mode function similar to standard 8051 port pins. A Quasi-
bidirectional port can be used both as an input and output without the need to reconfigure the
port. This is possible because when the port outputs a logic high, it is weakly driven, allowing an
external device to pull the pin low. When the pin is driven low, it is driven strongly and able to
sink a large current. There are three pull-up transistors in the quasi-bidirectional output that
serve different purposes.
One of these pull-ups, called the “very weak” pull-up, is turned on whenever the port latch for the
pin contains a logic “1”. This very weak pull-up sources a very small current that will pull the pin
high if it is left floating. When the pin is pulled low externally this pull-up will always source some
current.
A second pull-up, called the “weak” pull-up, is turned on when the port latch for the pin contains
a logic “1” and the pin itself is also at a logic “1” level. This pull-up provides the primary source
current for a quasi-bidirectional pin that is outputting a “1”. If this pin is pulled low by an external
device, this weak pull-up turns off, and only the very weak pull-up remains on. In order to pull the
pin low under these conditions, the external device has to sink enough current to overpower the
weak pull-up and pull the port pin below its input threshold voltage.
The third pull-up is referred to as the “strong” pull-up. This pull-up is used to speed up low-to-
high transitions on a quasi-bidirectional port pin when the port latch changes from a logic “0” to a
logic “1”. When this occurs, the strong pull-up turns on for one CPU clock, quickly pulling the port
pin high. The quasi-bidirectional port configuration is shown in
The input only port configuration is shown in
input includes a Schmitt-triggered input for improved input noise rejection. The input circuitry of
P3.2, P3.3, P4.6 and P4.7 is not disabled during Power-down (see
these pins should not be left floating during Power-down when configured in this mode.
Input-only mode can reduce power consumption for low-level inputs over quasi-bidirectional
mode because the “very weak” pull-up is turned off and only very small leakage current in the
sub microamp range is present.
P3M0
6
P2M1
5
P2M0
4
P1M1
3
Figure
10-2. The output drivers are tristated. The
P1M0
2
Figure
Reset Value = 0000 0011B
P0M1
Figure
10-1.
1
10-3) and therefore
3709D–MICRO–12/11
P0M0
0