CS494502 datasheet(48/100 Pages) CIRRUS

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5) If INTREQ is still low after 4 bytes, another 4

bytes should be clocked out of DSPC.

6) When INTREQ is released, the chip select line

of DSPC should be taken high to end the read

transaction.

All messages read back from DSPC will be in 4-

byte multiples.

6.1.3 FINTREQ Behavior: A Special Case

When communicating with DSPAB there are two

types of messages which force FINTREQ to go

low. These messages are known as solicited

messages and unsolicited messages. For more

information on the specific types of messages that

require a read from the host, one of the application

code user’s guides should be referenced.

In general, when communicating with DSPAB,

FINTREQ will not go low unless the host first

sends a read request command message. In other

words the host must solicit a response from the

DSP. In this environment, the host must read from

DSPAB until FINTREQ goes high again. Once the

FINTREQ pin has gone high it will not be driven

low until the host sends another read request.

When unsolicited messages, such as those used for

Autodetect, have been enabled, the behavior of

FINTREQ is noticeably different. DSPAB will

drop the FINTREQ pin whenever it has an

outgoing message, even though the host may not

have requested data.

There are three ways in which FINTREQ can be

affected by an unsolicited message:

1) During normal operation, while FINTREQ is

high, DSPAB could drop FINTREQ to indicate an

outgoing message, without a prior read request.

2) The host is in the process of reading from

DSPAB, meaning that FINTREQ is already low.

An unsolicited message arrives which forces

FINTREQ to remain low after the solicited

message is read.

3) The host is reading from DSPAB when the

unsolicited message is queued, but FINTREQ goes

high for one period of FSCCLK and then goes low

again before the end of the read cycle.

In case (1) the host should perform a read operation

as discussed in the previous sections.

In case (2) an unsolicited message arrives before

the second to last FSCCLK of the final byte

transfer of a read, forcing the FINTREQ pin to

remain low. In this scenario the host should

continue to read from DSPAB without a stop/start

condition or data will be lost.

In case (3) an unsolicited message arrives between

the second to last FSCCLK and the last FSCCLK

of the final byte transfer of a read. In this scenario,

FINTREQ will transition high for one clock (as if

the read transaction has ended), and then back low

(indicating that more data has queued). This final

case is the most complicated and shall be explained

in detail.

There are two constraints which completely

characterize the behavior of the FINTREQ pin

during a read. The first constraint is that the

FINTREQ pin is guaranteed to remain low until the

second to last FSCCLK (FSCCLK number N-1) of

the final byte being transferred from DSPAB (not

necessarily the second to last bit of the data byte).

The second constraint is that once the FINTREQ

pin has gone high it is guaranteed to remain high

until the rising edge of the last FSCCLK (FSCCLK

number N) of the final byte being transferred from

DSPAB (not necessarily the last bit of the data

byte). If an unsolicited message arrives in the

window of time between the rising edge of the

second to last FSCCLK and the final FSCCLK,

FINTREQ will drop low on the rising edge of the

final FSCCLK as illustrated in the functional

timing diagram shown for the SPI read cycle.

FINTREQ behavior for SPI communication is

illustrated in Figure 31, "SPI Timing for DSPAB"

on page 43. When using SPI communication, the

FINTREQ pin will remain low until the rising edge

of FSCCLK for the data bit D1 (FSCCLK N-1), but

it can go low at the rising edge of FSCCLK for data

bit D0 (FSCCLK N) if an unsolicited message has

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