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AD7621

Rev. 0 | Page 26 of 32

MICROPROCESSOR INTERFACING

The AD7621 is ideally suited for traditional dc measurement

applications supporting a microprocessor, and ac signal

processing applications interfacing to a digital signal processor.

The AD7621 is designed to interface with a parallel 8-bit or

16-bit wide interface, or with a general-purpose serial port or

I/O ports on a microcontroller. A variety of external buffers can

be used with the AD7621 to prevent digital noise from coupling

into the ADC. The following section illustrates the use of the

AD7621 with an ADSP-219x SPI-equipped DSP.

SPI Interface (ADSP-219x)

Figure 42 shows an interface diagram between the AD7621 and

an SPI-equipped DSP, ADSP-219x. To accommodate the slower

speed of the DSP, the AD7621 acts as a slave device and data

must be read after conversion. This mode also allows the daisy-

chain feature. The convert command could be initiated in

response to an internal timer interrupt. The reading process can

be initiated in response to the end-of-conversion signal (BUSY

going low) using an interrupt line of the DSP. The serial

peripheral 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

by writing to the SPI control register (SPICLTx). It should be

noted that to meet all timing requirements, the SPI clock should

be limited to 17 Mb/s allowing it to read an ADC result in less

than 1 μs. When a higher sampling rate is desired, use one of

the parallel interface modes.

BUSY

SDOUT

SCLK

CNVST

AD7621*

PFx

SPIxSEL (PFx)

MISOx

SCKx

PFx OR TFSx

ADSP-219x*

*ADDITIONAL PINS OMITTED FOR CLARITY

DVDD

SER/PAR

EXT/INT

INVSCLK

Figure 42. Interfacing the AD7621 to SPI Interface

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