Datasheet

AD7654
Rev. B | Page 24 of 28
MICROPROCESSOR INTERFACING
The AD7654 is ideally suited for traditional dc measurement
applications supporting a microprocessor and for ac signal
processing applications interfacing to a digital signal processor.
The AD7654 is designed to interface with either a parallel 8-bit
wide or 16-bit wide interface, a general-purpose serial port, or
I/O ports on a microcontroller. A variety of external buffers can
be used with the AD7654 to prevent digital noise from coupling
into the ADC. The following section illustrates the use of the
AD7654 with an SPI-equipped DSP, the ADSP-219x.
SPI INTERFACE (ADSP-219X)
Figure 34 shows an interface diagram between the AD7654 and
the SPI equipped ADSP-219x. To accommodate the slower
speed of the DSP, the AD7654 acts as a slave device and data
must be read after conversion. This mode also allows the daisy-
chain feature. The convert command can be initiated in
response to an internal timer interrupt. The 32-bit output data
is read with two serial peripheral interface (SPI) 16-bit wide
accesses. The reading process can be initiated in response to the
end-of-conversion signal (BUSY going low) using an
interrupt line of the DSP. By writing to the SPI control register
(SPICLTx), the serial interface (SPI) on the ADSP-219x is
configured for master mode (MSTR) = 1, clock polarity bit
(CPOL) = 0, clock phase bit (CPHA) = 1, and SPI interrupt
enable (TIMOD) = 00. To meet all timing requirements, the SPI
clock should be limited to 17 Mbps, which allows it to read an
ADC result in less than 1 µs. When a higher sampling rate is
desired, use of one of the parallel interface modes is
recommended.
AD7654*
ADSP-219x*
SER/PAR
PFx
MISOx
SCKx
PFx or TFSx
BUSY
SDOUT
SCLK
CNVST
EXT/INT
CS
RD
INVSCLK
D
V
DD
*ADDITIONAL PINS OMITTED FOR CLARITY
SPIxSEL (PFx)
0
3057-034
Figure 34. Interfacing the AD7654 to an SPI Interface