P89LPC915_916_917_5 © NXP B.V. 2009. All rights reserved.
Product data sheet Rev. 05 — 15 December 2009 36 of 75
NXP Semiconductors
P89LPC915/916/917
8-bit microcontrollers with accelerated two-clock 80C51 core
8.13 I/O ports
The P89LPC916 and P89LPC917 devices have three I/O ports: Port 0, Port 1, and Port 2.
The exact number of I/O pins available depends upon the clock and reset options chosen,
as shown in Table 11.
[1] Required for operation above 12 MHz.
The P89LPC915 has two I/O ports: Port 0 and Port 1. The exact number of I/O pins
available depends upon the clock and reset options chosen, as shown in Table 12.
[1] Required for operation above 12 MHz.
8.13.1 Port configurations
All but three I/O port pins on the P89LPC915/916/917 may be configured by software to
one of four types on a bit-by-bit basis. These are: quasi-bidirectional (standard 80C51 port
outputs), push-pull, open drain, and input-only. Two configuration registers for each port
select the output type for each port pin.
1. P1.5 (RST) can only be an input and cannot be configured.
2. P1.2 (SCL/T0) and P1.3 (SDA/INT0) may only be configured to be either input-only or
open-drain.
8.13.1.1 Quasi-bidirectional output configuration
Quasi-bidirectional 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 driven
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.
Table 11. Number of I/O pins available (P89LPC916 and P89LPC917)
Clock source Reset option Number of I/O
pins (16-pin
package)
RC oscillator or watchdog
oscillator
No external reset (except during
power-up)
14
External RST pin supported 13
External clock input No external reset (except during
power-up)
13
External
RST pin supported
[1]
12
Table 12. Number of I/O pins available (P89LPC915)
Clock source Reset option Number of I/O
pins (14-pin
package)
RC oscillator or watchdog
oscillator
No external reset (except during
power-up)
12
External RST pin supported 11
External clock input No external reset (except during
power-up)
11
External
RST pin supported
[1]
10
