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Производитель: Philips Размер: 671.08 kb Название файла: user_manual_p89lpc901_902_903.pdf Язык инструкции:

I/O PORTS The P89LPC901/902/903 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 Clock Source Reset Option Number of I/O Pins 8-Pin Package On-chip oscillator or watchdog No external reset(except during power-up) 6 oscillator External RST pin supported 5 External clock input No external reset(except during power-up) 5 External RST pin supported 4 Low/medium/high speed oscillator (external crystal or resonator) (P89LPC901) No external reset(except during power-up) 4 External RST pin supported 3 Port Configurations All but one I/O port pin on the P89LPC901/902/903 may be configured by software to one of four types on a pin-by-pin basis, as shown in Table . 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 39 Philips Semiconductors User’s Manual - Preliminary I/O PORTS P89LPC901/902/903 The third pullup is referred to as the "strong" pullup. This pullup 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 pullup turns on for two CPU clocks quickly pulling the port pin high . The quasi-bidirectional port configuration is shown in Figure 4-1. Although the P89LPC901/902/903 is a 3V device the pins are 5V-tolerant (except for XTAL1 and XTAL2). If 5V is applied to a pin configured in quasi-bidirectional mode, there will be a current flowing from the pin to VDD causing extra power consumption. Therefore, applying 5V to pins configured in quasi-bidirectional mode is discouraged. A quasi-bidirectional port pin has a Schmitt-triggered input that also has a glitch suppression circuit. (Please refer to the P89LPC901/902/903 datasheet, AC Characteristics for glitch filter specifications) weak very weak strong port pin VDD 2 CPU clock delay input data port latch data VDD VDD glitch rejection Figure 4-1: Quasi-Bidirectional Output Open Drain Output Configuration The open drain output configuration turns off all pullups and only drives the pulldown transistor of the port pin when the port latch contains a logic 0. To be used as a logic output, a port configured in this manner must have an external pullup, typically a resistor tied to VDD. The pulldown for this mode is the same as for the quasi-bidirectional mode. The open drain port configuration is shown in Figure 4-2. An open drain port pin has a Schmitt-triggered input that also has a glitch suppression circuit .(Please refer to the P89LPC901/ 902/903 datasheet, AC Characteristics for glitch filter specifications). port pin input data port latch data glitch rejection Figure 4-2: Open Drain Output 2003 Dec 8 40 Philips Semiconductors User’s Manual - Preliminary I/O PORTS P89LPC901/902/903 Input-Only Configuration The input port configuration is shown in Figure 4-3. It is a Schmitt-triggered input that also has a glitch suppression circuit. (Please refer to the P89LPC901/902/903 datasheet, AC Characteristics for glitch filter specifications) port pin input data glitch rejection Figure 4-3: Input Only Push-Pull Output Configuration The push-pull output configuration has the same pulldown structure as both the open drain and the quasi-bidirectional output modes, but provides a continuous strong pullup when the port latch contains a logic 1. The push-pull mode may be used when...

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