AD7921AUJ-R2 datasheet(23/28 Pages) AD

23 / 28 page

AD7911/AD7921

Rev. 0 | Page 23 of 28

For example, the ADSP-2189 has a master clock frequency of

40 MHz. If the SCLKDIV register is loaded with the value of 3,

then an SCLK of 5 MHz is obtained, and eight master clock

periods elapse for every one SCLK period. Depending on the

throughput rate selected, if the timer register is loaded with the

value 803 (803 + 1 = 804), then 100.5 SCLK occur between

interrupts and subsequently between transmit instructions. This

situation results in nonequidistant sampling, because the

transmit instruction occurs on a SCLK edge. If the number of

SCLKs between interrupts is a whole integer figure of N, then

equidistant sampling is implemented by the DSP.

AD7911/AD7921 to DSP563xx Interface

The connection diagram in Figure 34 shows how the AD7911/

AD7921 can be connected to the SSI (synchronous serial

interface) of the DSP563xx family of DSPs from Motorola. The

SSI is operated in synchronous and normal mode (SYN = 1 and

MOD = 0 in the Control Register B, CRB) with internally

generated word frame sync for both Tx and Rx (Bits FSL1 = 0

and FSL0 = 0 in the CRB). Set the word length in the Control

WL0 = 0 for the AD7921. This DSP does not offer the option for

a 14-bit word length, so the AD7911 word length is set up to

16 bits like the AD7921. For the AD7911, the conversion process

uses 16 SCLK cycles, with the last two clock periods clocking

out two trailing zeros to fill the 16-bit word.

To implement the power-down mode on the AD7911/AD7921,

the word length can be changed to 8 bits by setting Bits

WL2 = 0, WL1 = 0, and WL0 = 0 in CRA. The FSP bit in the

CRB register can be set to 1, which means that the frame goes

low and a conversion starts. Likewise, by means of the Bits

SCD2, SCKD, and SHFD in the CRB register, the Pin SC2 (the

frame sync signal) and SCK in the serial port are configured as

outputs, and the MSB is shifted first.

The values are as follows:

MOD = 0

SYN = 1

WL2, WL1, WL0 depend on the word length

FSL1 = 0, FSL0 = 0

FSP = 1, negative frame sync

SCD2 = 1

SCKD = 1

SHFD = 0

Note that, for signal processing applications, the frame

synchronization signal from the DSP563xx must provide

equidistant sampling.

AD7911/

AD7921*

DSP563xx*

SCK

SCLK

SRD

DOUT

STD

DIN

SC2

*ADDITIONAL PINS OMITTED FOR CLARITY

Figure 34. Interfacing to the DSP563xx

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