13 Hardware Examples
13 – 10
13.5
SERIAL PORT TO ADC INTERFACE
An ADC (analog-to-digital converter) converts an analog signal to digital
samples that a DSP processor can operate on. The ADSP-21xx processors
can receive data from an ADC directly through a serial port.
Analog Devices’ AD7872 is an ADC that requires no extra logic to
interface to the SPORT. The AD7872 converts an analog signal to 14-bit
samples. Each sample is padded with two zero MSBs to yield 16-bit
samples. The AD7872 outputs each sample serially, MSB first. Its digital
interface consists of three pins: SDATA, the serial data output; SCLK, for
clocking data out; and
SSTRB
, (serial strobe), which frames each serial
word.
The serial port connection to the AD7872 is shown in Figure 13.6. The
timer regulates sampling via the
CONVST
input at a constant frequency.
Instead of the timer, an unused serial clock or flag output from the ADSP-
21xx processor can be programmed to generate the
CONVST
signal. The
AD7872 generates SCLK internally and provides it to the processor. With
the CONTROL input held at –5 V, the SCLK signal is continuous, running
even when no data is being output.
ADSP-21xx
RFS
DR
SCLK
SPORT
SSTRB
SDATA
SCLK
AD7872
CONTROL
–5 V
CONVST
TIMER
+5 V
4.7 k
Ω
4.7 k
Ω
2 k
Ω
Figure 13.6 Serial Port Interface To AD7872 ADC
Serial data is output from the SDATA output of the ADC to the
processor’s DR pin. The
SSTRB
signal provides the RFS input to the
processor.
SSTRB
goes low when the first bit is transmitted to the
processor. Figure 13.7 shows the timing of the serial data transfer.