Specifying The Dac Output Voltage; Ucf Location Constraints; Related Resources - Xilinx Spartan-3A User Manual

Chapter 10: Digital-to-Analog Converter (DAC)
The FPGA first sends eight dummy or don't care bits, followed by a four-bit command. The
most commonly used command with the board is COMMAND[3:0] = 0011 binary, 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 four-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
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
voltage, V
3.3V reference voltage. Channels C and D have a separate reference voltage, nominally also
3.3V, supplied by the LP3906 regulator designated as IC18. The reference voltage for
Channels C and D can be modified, as described in
page
The reference voltages themselves have a ±5% tolerance, so there are slight corresponding
variances in the output voltage.

UCF Location Constraints

Figure 10-5
assignment and the I/O standard used.

Related Resources

Refer to the following links for additional information:
•
•
•
82
Figure 10-2, each DAC output level is the analog equivalent of a 12-bit
, is different between the four DAC outputs. Channels A and B use a
REFERENCE
140.
V
=
OUT
provides the UCF constraints for the DAC interface, including the I/O pin
NET "SPI_MOSI" LOC = "AB14" |
NET "SPI_SCK" LOC = "AA20" |
NET "DAC_CS" LOC = "W7"
NET "DAC_CLR" LOC = "AB13" |
NET "DAC_OUT" LOC = "V7"
Figure 10-5: UCF Location Constraints for the DAC Interface
LTC2624 Quad DAC Data Sheet
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1155,C1005,C1156,P2048,D2170
Xilinx PicoBlaze Soft Processor
http://www.xilinx.com/picoblaze
Digilent, Inc. Peripheral Modules
http://www.digilentinc.com/Products/Catalog.cfm?Nav1=Products&Nav2=Peripheral&Cat=Peripheral
www.xilinx.com
2
"I C Voltage Adjustment Interface,"
[ ]
D 11:0
×
-------------------- -
V
,
REFERENCE
4 096
IOSTANDARD = LVTTL |
IOSTANDARD = LVTTL |
| IOSTANDARD = LVTTL |
IOSTANDARD = LVTTL |
| IOSTANDARD = LVTTL ;
Spartan-3A/3AN Starter Kit Board User Guide
Equation 10-1. The reference
Equation 10-1
SLEW = SLOW | DRIVE = 4 ;
SLEW = SLOW | DRIVE = 4 ;
SLEW = SLOW | DRIVE = 4 ;
SLEW = SLOW | DRIVE = 4 ;
UG334 (v1.0) May 28, 2007
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