AD1871 datasheet(19/28 Pages) AD

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REV. 0

AD1871

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Cascade Mode

The AD1871 supports cascading of up to four devices in a

daisy-chain configuration to the serial port of a DSP. In Cascade

Mode, each device loads an internal 64-Bit Shift Register with

the results of the left and right channel conversions. The 64-

Bit Register is split into two subframes of 32 bits each; the first

for left channel data and the second for right channel data.

The results are left-justified, MSB first within the subframes,

and the word-width setting in Control Register II applies.

Remaining bits within the subframe, beyond the conversion

word-width, are set to zero. Please refer to Figure 16.

Up to four devices can be connected in a daisy chain as shown

in Figure 17. All devices must be set in Cascade Mode by tying

the CASC pin of each device to a logic high. The first device in

the chain (Device 4) has its DIN pin tied to logic low. Its

DOUT pin is connected to the DIN pin of Device 3 whose

DOUT is in turn connected to the DIN pin of Device 2. This

daisy chaining is continued until the DOUT of Device 1 is

connected to the DSP’s serial port RX data line (DR0). The

DSP’s RX serial clock (RXCLK0) is connected to the BCLK

pin of all AD1871 devices and the DSP’s RX frame sync (RFS0)

is connected to the LRCLK pin of all AD1871 devices.

64-BIT FRAME

32-BIT LEFT SUBFRAME

32-BIT RIGHT SUBFRAME

16-BIT RESULT

20-BIT RESULT

24-BIT RESULT

20-BIT RESULT

24-BIT RESULT

Figure 16. DSP Mode

The DSP can be the master and supply the frame sync and

serial clock to the AD1871s, or one of the AD1871s can be

set as the master with the DSP and all other AD1871s set to

slave. Each sampling period begins with a frame sync being gener-

ated either by the DSP or one of the AD1871s, depending on

the

Master/Slave selection. The frame-sync pulse causes each

device to load the 64-Bit Data I/O Register with the left and

right ADC results. These results are then clocked toward the

DSP where they are received in the following order: Device 1,

Left; Device 1, Right; Device 2, Left; Device 2, Right; Device 3,

Left; Device 3, Right; Device 4, Left; and Device 4, Right.

The DSP’s serial port must be programmed to accept 32-bit

word lengths regardless of the AD1871 word length. The number

of sample words to be accepted per sample interval will be

determined by the number of AD1871 devices in cascade, up

to a maximum of eight words corresponding with the maximum

number of four devices.

Figure 17 also shows the connection of a separate DSP serial port

interface to the control port (SPI) interface of the cascaded

AD1871s. Again this cascade is implemented as a daisy chain,

where the control words for the four devices are output in

sequence (depending on the hookup – 1, 2, 3, and 4 in the

example) to be latched simultaneously at each device by the

common CLATCH. In this mode, it is necessary to send a

control word for each device (16 bits

the number of devices)

from the SPI port of the control host. The CLATCH signal can

be controlled from a separate programmable output line. It is

also possible to have individual read/write of the AD1871s

using separate CLATCH controls for each device.

When using Cascade Mode, the data interface defaults to left-

justified, MSB first data, regardless of the state of the Interface

Mode selection (by SPI or external control).

The timing relationships of the Cascade Mode are shown in

Figure 18.

AD1871 No.1

AD1871 No.2

AD1871 No.3

AD1871 No.4

DT1

DR1

TXCLK1/RXCLK1

TFS1/RFS1

RFS0

RXCLK0

DR0

ADSP-21xxx

SHARC DSP

Figure 17. DSP Mode

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