P87LPC778FDH,529 Datasheet P87LPC778FDH,529 | P34-P36

Philips Semiconductors

P87LPC778

CMOS single-chip 8-bit microcontroller

Product data Rev. 01 — 31 March 2004 34 of 79

9397 750 12378

8.9 I/O ports

The P87LPC778 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 15 pins

of the P87LPC778 may be used as I/Os when a two-pin external oscillator and an

external reset circuit are used. Up to 18 pins may be available if fully on-chip oscillator

and reset conﬁgurations are chosen.

All but three I/O port pins on the P87LPC778 may be software conﬁgured to one of

four types on a bit-by-bit basis, as shown in Table 3 2. 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.

8.9.1 Quasi-bidirectional output conﬁguration

The default port output conﬁguration for standard P87LPC778 I/O ports is the

quasi-bidirectional output that is common on the 80C51 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

Fig 9. Interrupt sources, interrupt enables, and Power-down wake-up sources.

002aaa627

IE0

EX0

IE1

EX1

BOD

EBO

KBF

EKB

INTERRUPT

TO CPU

WAKE-UP

(IF IN POWER-

DOWN)

EC2

CM2

EWD

WDT

EAD

ADC

EC1

CM1

EA (from IE0 register)

TF0

ET0

TF1

ET1

RI & TI

ATN

EI2

TIMER 1 INTERRUPT

ETI

Table 32: 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

Philips Semiconductors

P87LPC778

CMOS single-chip 8-bit microcontroller

Product data Rev. 01 — 31 March 2004 35 of 79

9397 750 12378

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 quasi-bidirectional pin that is outputting a ‘1’. If a pin that

has a logic 1 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.

8.9.2 Open drain output conﬁguration

The open drain output conﬁguration turns off all pull-ups and only drives the pulldown

transistor of the port driver 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 pull-up, typically

a resistor tied to V

. The pulldown for this mode is the same as for the

quasi-bidirectional mode.

The open drain port conﬁguration is shown in Figure 11.

Fig 10. Quasi-bidirectional output.

002aaa628

2 CPU

CLOCK DELAY

port latch

data

weakstrong

input

data

very

weak

PP P

port

Philips Semiconductors

P87LPC778

CMOS single-chip 8-bit microcontroller

Product data Rev. 01 — 31 March 2004 36 of 79

9397 750 12378

8.9.3 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

pull-up when the port latch contains a logic 1. The push-pull mode may be used when

more source current is needed from a port output.

The push-pull port conﬁguration is shown in Figure 12.

The three port pins that cannot be conﬁgured are P1.2, P1.3, and P1.5. The port pins

P1.2 and P1.3 are permanently conﬁgured as open drain outputs. They may be used

as inputs by writing ones to their respective port latches. P1.5 may be used as a

Schmitt trigger input if the P87LPC778 has been conﬁgured for an internal reset and

is not using the external reset input function RST.

Additionally, port pins P2.0 and P2.1 are disabled for both input and output if one of

the crystal oscillator options is chosen. Those options are described in Section 8.10

“Oscillator” on page 39.

Fig 11. Open drain output.

002aaa629

port latch

data

input

data

port

Fig 12. Push-pull output.

002aaa630

port latch

data

input

data

port

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P87LPC778FDH,529

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IC MCU 8BIT 8KB OTP 20TSSOP

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