p87lpc761bdh NXP Semiconductors, p87lpc761bdh Datasheet - Page 21

• Manufacturer Part Number: p87lpc761bdh
• Description: Low Power, Low Price, Low Pin Count 16 Pin Microcontroller With 2 Kbyte Otp
• Manufacturer: NXP Semiconductors

Philips Semiconductors
I/O Ports
The P87LPC761 has 3 I/O ports, port 0, port 1, and port 2. The
exact number of I/O pins available depend upon the oscillator and
reset options chosen. At least 11 pins of the P87LPC761 may be
used as I/Os when a two-pin external oscillator and an external
reset circuit are used. Up to 14 pins may be available if fully on-chip
oscillator and reset configurations are chosen.
All but three I/O port pins on the P87LPC761 may be
software-configured to one of four types on a bit-by-bit basis, as
shown in Table 4. These are: quasi-bidirectional (standard 80C51
port outputs), push-pull, open drain, and input only. Two
configuration registers for each port choose the output type for each
port pin.
Table 4. Port Output Configuration Settings
Quasi-Bidirectional Output
The default port output configuration for standard P87LPC761 I/O
ports is the quasi-bidirectional output that is common on the 80C51
2002 Mar 07
Low power, low price, low pin count (16 pin)
microcontroller with 2 kbyte OTP
PxM1.y
0
0
1
1
PORT LATCH
PxM2.y
DATA
0
1
0
1
Configuration
Input Only (High Impedance)
Port Output Mode
Quasi-bidirectional
Open Drain
Push-Pull
CLOCK DELAY
2 CPU
Figure 10. Quasi-Bidirectional Output
18
quasi-bidirectional pin that is outputting a 1. If a pin that has a logic 1
and most of its derivatives. This output type can be used as both 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 pulled low, it is driven strongly and able to sink a fairly large
current. These features are somewhat similar to an open drain
output except that 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. The very weak
pull-up sources a very small current that will pull the pin high if it is
left floating.
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
on it is pulled low by an external device, the 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 take the voltage on the
port pin below its input threshold.
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 a brief time, two CPU clocks, in
order to pull the port pin high quickly. Then it turns off again.
The quasi-bidirectional port configuration is shown in Figure 10.
N
P
INPUT
DATA
STRONG
P
V
DD
VERY
WEAK
P
WEAK
P87LPC761
SU01159
PORT
PIN
Preliminary data