PC87570 datasheet(127/168 Pages) NSC

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Digital to Analog Converter (DAC)

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17.0 Digital to Analog Converter (DAC)

The DAC receives digital data and delivers analog signals

on four output pins. It includes four independent digital to

analog converters. Each of them has 8-bit resolution and a

full output range from AGND to AVCC. The DAC has a typi-

cal output impedance of 3 K

Ω, which allows a settling time

of about 1

µs on a 50 pF load.

17.1 FEATURES

• 8-bit resolution

• 4-channel D/A converter

• Fast settling time, 1 µs typical, on 50 pF capacitive load

• Output swing from AGND to AV

• Independent enable/disable for each channel

• Zero power when disabled, low power when enabled

• Outputs drive zero when disabled

17.2 FUNCTIONAL DESCRIPTION

The DAC comprises four independent digital to analog con-

verters. The converters drive the four output pins DA0-3, as

shown in Figure 17-1.

After reset (on power-up or when a positive pulse is applied

on the HMR pin), all four channels are disabled and the volt-

age on the DA0-3 outputs is 0 V.

When a DAC channel is enabled, its output is defined by the

value written to its DACDAT Register. DACDAT0 through

DACDAT3 control DA0 through DA3, respectively. The

maximum output voltage is (255/256)*AVCC and is obtained

for a value of FFh. The minimum output, AGND, is obtained

for a value of 00h.

The reference voltage of the converters is the AVCC analog

power supply voltage. This allows full swing of the outputs

from zero to nearly AVCC.

The Control Register is used to enable/disable each of the

four channels. The DAC can be disabled by software before

entering Idle mode.

17.2.1

DAC Reset

The DACCTRL Register is reset to its default value, in the

following two ways:

Cold Reset. Upon power-up, an internal power-up detect

circuit generates a reset cycle. See Section 2.3 on page 26.

Warm Reset. When the chip is powered up and a positive

pulse is applied on the HMR pin, a reset cycle is generated.

See HMR pin functionality in Table 2-2 on page 21.

17.2.2

Reference Voltage

The analog output voltages are converted relative to a ref-

erence voltage. See details in Figure 17-1 and Table 19-7

on page 135. The internally connected reference voltage of

the DAC is the analog power supply. To assure good signal

quality on the DAC outputs, a low noise analog power sup-

ply should be used.

17.2.3

Output Signal Range

The DAC performs a linear conversion of the input digital

value written into the DACDATA0-3 Registers to an un-

signed analog output signal. The output signal should be

taken relative to the analog ground pin (AGND), and can

range from a minimum of AGND to almost AVCC. See

Table 19-6 on page 134.

A 00h written to the DACDAT0-3 Registers results in an out-

put signal of zero (ground) for the respective output. An FFh

written to the DACDAT0-3 Registers results in an output

signal of (255/256)*AVCC for the respective output.

Peripheral

Figure 17-1. DAC Functional Diagram

DAC0

DA0

DA1

DA2

DA3

AVCC

VREF

AGND

0.1

DAC

DACDATA0

Bus

PC87570

Analog

Power

µF

3.3 V

5.0 V

AVCC

to other

modules

µF

DAC

Control

DAC1

VREF

DACDATA1

DAC2

VREF

DACDATA2

DAC3

VREF

DACDATA3

IDLE

AVCC

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