P87LPC779 datasheet(12/74 Pages) PHILIPS

12 / 74 page

Philips Semiconductors

P87LPC779

CMOS single-chip 8-bit microcontroller

Product data

Rev. 02 — 03 May 2004

12 of 74

9397 750 13213

Functional description

Remark: Please refer to the

P87LPC779 User’s Manual for a more detailed

functional description.

8.1 Enhanced CPU

The P87LPC779 uses an enhanced 80C51 CPU which runs at twice the speed of

standard 80C51 devices. This means that the performance of the P87LPC779

running at 5 MHz is exactly the same as that of a standard 80C51 running at 10 MHz.

A machine cycle consists of 6 oscillator cycles, and most instructions execute in 6 or

12 clocks. A user conﬁgurable option allows restoring standard 80C51 execution

timing. In that case, a machine cycle becomes 12 oscillator cycles.

In the following sections, the term ‘CPU clock’ is used to refer to the clock that

controls internal instruction execution. This may sometimes be different from the

externally applied clock, as in the case where the part is conﬁgured for standard

80C51 timing by means of the CLKR conﬁguration bit or in the case where the clock

is divided down via the setting of the DIVM register. These features are described in

the Section 8.10 “Oscillator” on page 34.

8.2 Analog functions

The P87LPC779 incorporates analog peripheral functions: an ADC, two Analog

Comparators, and two DACs. In order to give the best analog function performance

and to minimize power consumption, pins that are actually being used for analog

functions must have the digital outputs (except for the DAC output pins) and the

digital inputs must also be disabled.

Digital outputs are disabled by putting the port output into the Input Only (high

impedance) mode as described in the Section 8.9 “I/O ports” on page 29.

Digital inputs on port 0 may be disabled through the use of the PT0AD register. Each

bit in this register corresponds to one pin of Port 0. Setting the corresponding bit in

PT0AD disables that pin’s digital input. Port bits that have their digital inputs disabled

will be read as ‘0’ by any instruction that accesses the port.

8.3 Analog to digital converter

The P87LPC779 incorporates a four channel, 8-bit A/D converter. The A/D inputs are

alternate functions on four port 0 pins. Because the device has a very limited number

of pins, the A/D power supply and references are shared with the processor power

pins, VDD and VSS. The A/D converter operates down to a VDD supply of 3.0 V.

The A/D converter circuitry consists of a 4-input analog multiplexer and an 8-bit

successive approximation ADC. The A/D employs a ratiometric potentiometer which

guarantees DAC monotonicity.

The A/D converter is controlled by the special function register ADCON. Details of

ADCON are shown in Tables 4 and 5. The A/D must be enabled by setting the

ENADC bit at least 10 microseconds before a conversion is started, to allow time for

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