P87LPC779 datasheet(17/74 Pages) PHILIPS

17 / 74 page

Philips Semiconductors

P87LPC779

CMOS single-chip 8-bit microcontroller

Product data

Rev. 02 — 03 May 2004

17 of 74

9397 750 13213

ADRead:

orl

ADCON,A

; Add in the new channel number.

setb ADCS

; Start A/D conversion.

ADChk:

jnb

ADCI,ADChk

; Wait for ADCI to be set.

mov

A,DAC0

; Get A/D result.

clr

ADCI

; Clear the A/D completion flag.

anl

ADCON,#0fch

; Clear the A/D channel number.

ret

8.5 Digital to Analog Converter (DAC) outputs

The P87LPC779 provides a two channel, 8-bit DAC function. DAC0 is also part of the

A/D Converter and is should not be enabled while the A/D is active. The DAC outputs

function down to a VDD of 3.0 V. Digital outputs must be disabled on the DAC output

pins while the corresponding DAC is enabled, as described in Section 8.2 “Analog

functions” on page 12.

The DACs use the power supply as the references: VDD as the upper reference and

VSS as the lower reference. The DAC output is generated by a tap from a resistor

ladder and is not buffered. The maximum resistance to VDD or VSS from a DAC output

is 10 k

Ω. Care must be taken with the loading of the DAC outputs in order to avoid

distortion of the output voltage. DAC accuracy is affected by noise generated on-chip

and elsewhere in the application. Since the P87LPC779 power pins are used for the

DAC references, the power supply also affects the accuracy of the DAC outputs.

The ideal DAC output may be calculated as follows:

(2)

where DAC Value is the contents of the relevant DAC register: DAC0 or DAC1.

8.6 Analog comparators

Two analog comparators are provided on the P87LPC779. Input and output options

allow use of the comparators in a number of different conﬁgurations. Comparator

operation is such that the output is a logic 1 (which may be read in a register and/or

routed to a pin) when the positive input (one of two selectable pins) is greater than the

Fig 6.

DAC block diagram.

Result

DAC value

0.5

()

–

256

--------------------------

D/A CONVERTER

REF

+ = V

ADCON

DACn

REF

- = V

DACn pin

ENDACn

002aaa817

(D/A conversion, write DACn)

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