CS494502 datasheet(49/100 Pages) CIRRUS

49 / 100 page

arrived. If no unsolicited messages arrive, the

FINTREQ pin will remain high after rising.

Ideally, the host will sample FINTREQ on the

falling edge of FSCCLK number N-1 of the final

byte of each read response message. If FINTREQ

is sampled high, the host should conclude the

current read cycle using the stop condition defined

for the communication mode chosen. The host

should then begin a new read cycle complete with

the appropriate start condition and the chip address.

If FINTREQ is sampled low, the host should

continue reading the next message from DSPAB

without ending the current read cycle.

When using automated communication ports,

however, the host is often limited to sampling the

status of FINTREQ after an entire byte has been

transferred. In this situation a low-high-low

transition (case 3) would be missed and the host

will see a constantly low FINTREQ pin. Since the

host should read from DSPAB until it detects that

FINTREQ has gone high, this condition will be

treated as a multiple-message read (more than one

read response is provided by DSPAB). Under these

conditions a single byte of 0x00 will be read out

before the unsolicited message.

The length of every read response is defined in the

user’s manual for each piece of application code.

Thus, the host should know how many bytes to

expect based on the first byte (the OPCODE) of a

read response message. It is guaranteed that no read

responses will begin with 0x00, which means that a

NULL byte (0x00) detected in the OPCODE

position of a read response message should be

discarded. Please see an Application Code User’s

Guide for an explanation of the OPCODE.

It is important that the host be aware of the

presence of NULL bytes, or the communication

channel could become corrupted.

When case (3) occurs and the host issues a stop

condition before starting a new read cycle, the first

byte of the unsolicited message is loaded directly

into the shift register and 0x00 is never seen.

Alternatively, if case (3) occurs and the host

continues to read from DSPAB without a stop

condition (a multiple message read), the 0x00 byte

must be shifted out of DSPAB before the first byte

of the unsolicited message can be read.

In other words, if a host can only sample FINTREQ

after an entire byte transfer the following routine

should be used if FINTREQ is low after the last

byte of the message being read:

1) Read one byte

2) If the byte = 0x00 discard it and skip to step 3.

If the byte != 0x00 then it is the OPCODE for

the next message. For this case skip to step 4.

3) Read one more byte. This is the OPCODE for

the next message.

4) Read the rest of the message as indicated in the

previous sections.

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