DAC104S085EVAL NSC [National Semiconductor], DAC104S085EVAL Datasheet - Page 17

• Manufacturer Part Number: DAC104S085EVAL
• Description: 10-Bit Micro Power QUAD Digital-to-Analog Converter with Rail-to-Rail Output
• Manufacturer: NSC [National Semiconductor]

2.0 Applications Information
2.3 DSP/MICROPROCESSOR INTERFACING
Interfacing the DAC104S085 to microprocessors and DSPs
is quite simple. The following guidelines are offered to has-
ten the design process.
2.3.1 ADSP-2101/ADSP2103 Interfacing
Figure
DAC104S085 and the ADSP-2101/ADSP2103. The DSP
should be set to operate in the SPORT Transmit Alternate
Framing Mode. It is programmed through the SPORT control
register and should be configured for Internal Clock Opera-
tion, Active Low Framing and 16-bit Word Length. Transmis-
sion is started by writing a word to the Tx register after the
SPORT mode has been enabled.
2.3.2 80C51/80L51 Interface
A serial interface between the DAC104S085 and the 80C51/
80L51 microcontroller is shown in Figure 11. The SYNC
signal comes from a bit-programmable pin on the microcon-
troller. The example shown here uses port line P3.3. This line
is taken low when data is transmitted to the DAC104S085.
Since the 80C51/80L51 transmits 8-bit bytes, only eight
falling clock edges occur in the transmit cycle. To load data
into the DAC, the P3.3 line must be left low after the first
eight bits are transmitted. A second write cycle is initiated to
transmit the second byte of data, after which port line P3.3 is
brought high. The 80C51/80L51 transmit routine must rec-
ognize that the 80C51/80L51 transmits data with the LSB
first while the DAC104S085 requires data with the MSB first.
2.3.3 68HC11 Interface
A serial interface between the DAC104S085 and the
68HC11 microcontroller is shown in Figure 12. The SYNC
line of the DAC104S085 is driven from a port line (PC7 in the
figure), similar to the 80C51/80L51.
The 68HC11 should be configured with its CPOL bit as a
zero and its CPHA bit as a one. This configuration causes
data on the MOSI output to be valid on the falling edge of
(Continued)
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FIGURE 10. ADSP-2101/2103 Interface
FIGURE 11. 80C51/80L51 Interface
shows
a
serial
interface
between
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SCLK. PC7 is taken low to transmit data to the DAC. The
68HC11 transmits data in 8-bit bytes with eight falling clock
edges. Data is transmitted with the MSB first. PC7 must
remain low after the first eight bits are transferred. A second
write cycle is initiated to transmit the second byte of data to
the DAC, after which PC7 should be raised to end the write
sequence.
2.3.4 Microwire Interface
Figure 13 shows an interface between a Microwire compat-
ible device and the DAC104S085. Data is clocked out on the
rising edges of the SK signal. As a result, the SK of the
Microwire device needs to be inverted before driving the
SCLK of the DAC104S085.
2.4 LAYOUT, GROUNDING, AND BYPASSING
For best accuracy and minimum noise, the printed circuit
board containing the DAC104S085 should have separate
analog and digital areas. The areas are defined by the
locations of the analog and digital power planes. Both of
these planes should be located in the same board layer.
There should be a single ground plane. A single ground
plane is preferred if digital return current does not flow
through the analog ground area. Frequently a single ground
plane design will utilize a "fencing" technique to prevent the
mixing of analog and digital ground current. Separate ground
planes should only be utilized when the fencing technique is
inadequate. The separate ground planes must be connected
in one place, preferably near the DAC104S085. Special care
is required to guarantee that digital signals with fast edge
rates do not pass over split ground planes. They must al-
ways have a continuous return path below their traces.
The DAC104S085 power supply should be bypassed with a
10µF and a 0.1µF capacitor as close as possible to the
device with the 0.1µF right at the device supply pin. The
10µF capacitor should be a tantalum type and the 0.1µF
capacitor should be a low ESL, low ESR type. The power
supply for the DAC104S085 should only be used for analog
circuits.
Avoid crossover of analog and digital signals and keep the
clock and data lines on the component side of the board. The
clock and data lines should have controlled impedances.
FIGURE 13. Microwire Interface
FIGURE 12. 68HC11 Interface
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