AD73360 datasheet(23/35 Pages) AD

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REV. A

AD73360

–23–

Cascade Operation

The AD73360 has been designed to support up to eight devices

in a cascade connected to a single serial port (see Figure 17).

The SPORT interface protocol has been designed so that device

addressing is built into the packet of information sent to the

device. This allows the cascade to be formed with no extra hard-

ware overhead for control signals or addressing. A cascade can

be formed in either of the two modes previously discussed.

TFS

RFS

AD73360

SDIFS

SDI

SCLK

SDO

SDOFS

SCLK

DEVICE 1

MCLK

RESET

AD73360

74HC74

FL0

FL1

ADSP-2181

DSP

CLK

SDIFS

SDI

SCLK

SDO

SDOFS

DEVICE 2

MCLK

RESET

Figure 17. Connection of Two AD73360s Cascaded to

ADSP-2181

There may be some restrictions in cascade operation due to the

number of devices configured in the cascade and the serial clock

rate chosen. The formula below gives an indication of whether

the combination of sample rate, serial clock and number of

devices can be successfully cascaded. This assumes a directly

coupled frame sync arrangement as shown in Figure 12 and does

not take any interrupt latency into account.

Device Count

SCLK

≥

×− ×

[((

)

]

When using the indirectly coupled frame sync configuration in

cascaded operation it is necessary to be aware of the restrictions

in sending control word data to all devices in the cascade. The

user should ensure that there is sufficient time for all the control

words to be sent between reading the last ADC sample and the

start of the next sample period.

In Cascade Mode, each device must know the number of de-

vices in the cascade to be able to output data at the correct

time. Control Register A contains a 3-bit field (DC0–2) that is

programmed by the DSP during the programming phase. The

default condition is that the field contains 000b, which is equiva-

lent to a single device in cascade (see Table XVIII). However,

for cascade operation this field must contain a binary value that

is one less than the number of devices in the cascade. With a

number of AD73360s in cascade each device takes a turn to

send an ADC result to the DSP. For example, in a cascade of

two devices the data will be output as Device 2-Channel 1,

Device 1-Channel 1, Device 2-Channel 2, Device 1-Channel 2

etc. When the first device in the cascade has transmitted its

channel data there is an additional SCLK period during which

the last device asserts its SDOFS as it begins its transmission of

the next channel. This will not cause a problem for most DSPs

as they count clock edges after a frame sync and hence the

extra bit will be ignored.

When multiple devices are connected in cascade there are also

restrictions concerning which ADC channels can be powered

up. In all cases the cascaded devices must all have the same

channels powered up (i.e., for a cascade of two devices requir-

ing Channels 1 and 2 on Device 1 and Channel 5 on Device 2,

Channels 1, 2 and 5 must be powered up on both devices to

ensure correct operation). Figure 18 shows the timing se-

quence for two devices in cascade.

Table XVIII. Device Count Settings

DC2

DC1

DC0

Cascade Length

Connection of a cascade of devices to a DSP, as shown in

Figure 17, is no more complicated than connecting a single

device. Instead of connecting the SDO and SDOFS to the

DSP’s Rx port, these are now daisy-chained to the SDI and

SDIFS of the next device in the cascade. The SDO and

SDOFS of the final device in the cascade are connected to the

DSP’s Rx port to complete the cascade. SE and

RESET on all

devices are fed from the signals that were synchronized with

the MCLK using the circuit of Figure 19. The SCLK from

only one device need be connected to the DSP’s SCLK input(s)

as all devices will be running at the same SCLK frequency and

phase.

1234 5

6 7

8 910 11 12 13 14 15 16 1234 5

7 8 9 10 11 12 13 14 15 16 17

DEVICE 2 - CHANNEL 1

DEVICE 1 - CHANNEL 1

1234 5

6 7

DEVICE 2 - CHANNEL 2

Figure 18. Cascade Timing for a Two-Device Cascade

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