Low-Cost, Triple, 8-Bit Voltage-Output DACs

with Serial Interface

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11

Serial-Input Data Format and Control Codes

Table 2 lists the serial-input data format. The 16-bit

input word consists of an 8-bit control byte and an 8-bit

data byte. The 8-bit control byte is not decoded inter-

nally. Every control bit performs one function. Data is

clocked in starting with Q2 (uncommitted bit), followed

by the remaining control bits and the data byte. The

LSB of the data byte (B0) is the last bit clocked into the

shift register (Figure 2).

Example of a 16-bit input word:

The example above performs the following functions:

• 80hex (128 decimal) loaded into DAC registers

A and B.

• Content of the DAC C register remains unchanged.

• DAC A and DAC B are active.

• DAC C is shut down.

• LOUT is reset to 0.

Digital Inputs

The digital inputs are compatible with CMOS logic.

Supply current increases slightly when toggling the

logic inputs through the transition zone between

Digital Output

The latched digital output (LOUT) has a 1.6mA source

With a 1.6mA sink current, the output voltage is guaran-

teed to be no more than 0.4V. The output can be used

for digital auxiliary control. Please note that the digital

output remains fully active during shutdown mode.

Microprocessor Interfacing

The MAX512/MAX513 serial interface is compatible with

Microwire, SPI, and QSPI. For SPI and QSPI, clear the

CPOL and CPHA bits (CPOL = 0 and CPHA = 0).

CPOL = 0 sets the inactive state of clock to zero and

CPHA = 0 changes data at the falling edge of SCLK.

This setting allows both SPI and QSPI to run at full clock

speeds (0.5MHz and 4MHz, respectively). If a serial port

is not available on your µP, three bits of a parallel port

can be used to emulate a serial port by bit manipulation.

Minimize digital feedthrough at the voltage outputs by

operating the serial clock only when necessary.

CONTROL

DATA

Q2

Q1

SC

SB

SA

LC

LB

LA

B7

B4

B3

B2

B1

B0

FUNCTION

*

*

*

*

*

0

0

0

X

X

X

X

X

X

X

X

No Operation to DAC Registers

*

8-Bit DAC Data

Load Register to DAC C

*

8-Bit DAC Data

Load Register to DAC B

*

8-Bit DAC Data

Load Register to DAC A

*

8-Bit DAC Data

Load All DAC Registers

*

*

0

0

0

*

*

*

X

X

X

X

X

X

X

X

All DACs Active

*

*

1

0

0

*

*

*

X

X

X

X

X

X

X

X

Shut Down DAC C

*

*

0

1

0

*

*

*

X

X

X

X

X

X

X

X

Shut Down DAC B

*

*

0

0

1

*

*

*

X

X

X

X

X

X

X

X

Shut Down DAC A

*

*

1

1

1

*

*

*

X

X

X

X

X

X

X

X

Shut Down All DACs

X

0

*

*

*

*

*

*

X

X

X

X

X

X

X

X

Reset LOUT

X

1

*

*

*

*

*

*

X

X

X

X

X

X

X

X

Set LOUT

B5

B6

Table 2. Serial-Interface Programming Commands

X

Don’t care.

*

Not shown for clarity. The functions of loading and shutting down the DACs and programming the logic can be combined in a single command.

MSB

LSB

1

0

0

1

0

1

1

0

*

0

*

*

*

*

*

*

*

*

0

*

1

*

*

1

*

*

*

*

X

0

1

0

0

0

1

1

1

0

0

0

0

0

0

0

Q2 Q1 SC SB SA LC LB LA B7 B6 B5 B4 B3 B2 B1 B0

Loaded

Loaded

in First

in Last