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Specifying the dac output voltage, Dac outputs a and b, Figure 9-4 – Xilinx MIcroblaze Development Spartan-3E 1600E User Manual

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72

MicroBlaze Development Kit Spartan-3E 1600 Edition User Guide

www.xilinx.com

UG257 (v1.1) December 5, 2007

Chapter 9:

Digital to Analog Converter (DAC)

R

master. The response from the DAC can be ignored although it is a useful to confirm

correct communication.

The FPGA first sends eight dummy or “don’t care” bits, followed by a 4-bit command. The

most commonly used command with the board is COMMAND[3:0] = “0011”, which

immediately updates the selected DAC output with the specified data value. Following the

command, the FPGA selects one or all the DAC output channels via a 4-bit address field.

Following the address field, the FPGA sends a 12-bit unsigned data value that the DAC

converts to an analog value on the selected output(s). Finally, four additional dummy or
don’t care

bits pad the 32-bit command word.

Specifying the DAC Output Voltage

As shown in

Figure 9-2

, each DAC output level is the analog equivalent of a 12-bit

unsigned digital value, D[11:0], written by the FPGA to the DAC via the SPI interface.
The voltage on a specific output is generally described in

Equation 9-1

. The reference

voltage, V

REFERENCE

, is different between the four DAC outputs. Channels A and B use a

3.3V reference voltage and Channels C and D use a 2.5V reference. The reference voltages

themselves have a r5% tolerance, so there will be slight corresponding variances in the

output voltage.

Equation 9-1

DAC Outputs A and B

Equation 9-2

provides the output voltage equation for DAC outputs A and B. The

reference voltage associated with DAC outputs A and B is 3.3V r 5%.

Equation 9-2

Figure 9-4:

SPI Communications Protocol to LTC2624 DAC

0

31

x x

x x

x x

x x

x x

x x

9 10 11

8

7

6

5

4

3

2

1

0

ADDRESS

a3

a2

a1

a0

a

3

a

2

a

1

a

0

c3

c2

c1

c0

0 0 0 0
0 0 0 1
0 0 1 0
0 0 1 1

All

DAC A
DAC B
DAC C
DAC D

1 1 1 1

SPI_MOSI

SPI_SCK

DAC_CS

SPI_MISO

12-bit Unsigned

DATA

COMMAND

Don’t Care

UG257_09_04_060606

msb

lsb

Don’t Care

Slave: LTC2624 DAC

M

as

ter

S

pa

rt

an-

3E FPGA

V

OUT

D

11:0

>

@

4096

--------------------- V

REFERENCE

u

=

V

OUTA

D

11:0

>

@

4096

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

3.3V 5%

r

u

=