Philips P89LPC907, P89LPC906 Port Configurations, QUASI-BIDIRECTIONAL Output Configuration, Rst

Philips Semiconductors	User’s Manual - Preliminary -
I/O PORTS	P89LPC906/907/908
4. I/O PORTS

The P89LPC906/907/908 has between 3 and 6 I/O pins. The exact number of I/O pins available depends on the clock and reset options chosen:

Table 4-1: Number of I/O Pins Available

				Number of I/O
Clock Source	Reset Option			Pins
				8-Pin Package
On-chip oscillator or watchdog	No external reset(except during power-up)			6
oscillator	External		pin supported	5
		RST
External clock input	No external reset(except during power-up)			5
(P89LPC906)	External		pin supported	4
		RST
Low/medium/high speed oscillator	No external reset(except during power-up)			4
(external crystal or resonator)
	External RST pin supported			3
(P89LPC906)

PORT CONFIGURATIONS

All but one I/O port pin on the P89LPC906/907/908 may be configured by software to one of four types on a pin-by-pin basis, as shown in Table 4-2. 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. P1.5 (RST) can only be an input and cannot be configured.

Table 4-2: Port Output Configuration Settings

PxM1.y	PxM2.y	Port Output Mode
0	0	Quasi-bidirectional
0	1	Push-Pull
1	0	Input Only (High Impedance)
1	1	Open Drain

QUASI-BIDIRECTIONAL OUTPUT CONFIGURATION

Quasi-bidirectional outputs 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 pullup transistors in the quasi-bidirectional output that serve different purposes.

One of these pullups, called the "very weak" pullup, is turned on whenever the port latch for the pin contains a logic 1. This very weak pullup sources a very small current that will pull the pin high if it is left floating.

A second pullup, called the "weak" pullup, 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 pullup 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 pullup turns off, and only the very weak pullup remains on. In order to pull the pin low under these conditions, the external device has to sink enough current to overpower the weak pullup and pull the port pin below its input threshold voltage.

2003 Dec 8 35