13 Hardware Examples
13 – 8
13.4 SERIAL PORT TO DAC INTERFACE
Any DSP process must ultimately output analog information. The serial
port of the ADSP-21xx processors can send data directly to a DAC (digital-
to-analog converter) for conversion to an analog signal.
Analog Devices’ AD766 is a DAC that requires no extra logic to interface
to the SPORT. The AD766 receives 16-bit data words serially, MSB first,
which it then converts to an analog signal. Its digital interface consists of
three inputs: DATA, the serial data input;
CLK
, for clocking data into the
DAC (active low because data is clocked on the falling edge) and LE (latch
enable), which latches each 16-bit word into the conversion section of the
DAC.
The serial port connection to the AD766 is shown in Figure 13.3. In this
configuration, the processor generates SCLK internally and provides it to
the DAC. Serial data is output from the DT pin to the DATA input of the
DAC. The TFS signal provides the DAC’s LE input.
LE should go low on the clock cycle after the LSB (sixteenth bit) of a word
is transmitted, to latch the 16-bit word into the DAC. To provide this
timing, TFS is configured for the alternate framing mode, non-inverted; it
goes high when the first bit is transmitted and low after the last bit is
transmitted. This low-going edge latches the word into the AD766. The
only restriction is that the SPORT cannot transmit continuously; there
must be a break in between the last bit of one word and the first bit of the
next so that TFS can go low. Figure 13.4 shows the timing.
ADSP-21xx
TFS
DT
SCLK
SPORT
LE
DATA
CLK
AD766
VOUT
Figure 13.3 Serial Port Interface To AD766 DAC
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