CS8920A datasheet(86/144 Pages) CIRRUS

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5.0

OPERATION

5.1

Managing Interrupts and Servicing

the Interrupt Status Queue

The Interrupt Status Queue (ISQ) is used by the

CS8920A to communicate Event reports to the

host processor. Whenever an event occurs that

triggers an enabled interrupt, the CS8920A sets

the appropriate bit(s) in one of five registers,

maps the contents of that register to the ISQ, and

drives the selected interrupt request pin high (if

an earlier interrupt is waiting in the queue, the

interrupt request pin will already be high). When

the host services the interrupt, it must first read

the ISQ to learn the nature of the interrupt. It can

then process the interrupt (the first read to the

ISQ causes the interrupt request pin to go low.

Three of the registers mapped to the ISQ are

event registers: RxEvent (Register 4), TxEvent

(Register 8), and BufEvent (Register C). The

other two registers are counter-overflow reports:

RxMISS (Register 10) and TxCOL (Register 12).

There may be more than one RxEvent report

and/or more than one TxEvent report in the ISQ

at a time. However, there may be only one

BufEvent report, one RxMISS report and one

TxCOL report in the ISQ at a time.

Event reports stored in the ISQ are read out in

the order of priority, with RxEvent first, followed

by TxEvent, BufEvent, RxMiss, and then

TxCOL. The host only needs to read from one

location to get the interrupt currently at the front

of the queue. In Memory Mode, the ISQ is lo-

cated at PacketPage base + 0120h. In I/O Mode,

it is located at I/O base + 0008h. Each time the

host reads the ISQ, the bits in the corresponding

queue moves to the front.

When the host starts reading the ISQ, it must

read and process all Event reports in the queue.

A readout of a null word (0000h) indicates that

all interrupts have been read.

The ISQ is read as a 16-bit word. The lower six

bits (0 through 5) contain the register number (4,

8, C, 10, or 12). The upper ten bits (6 through F)

contain the register contents. The host must al-

ways read the entire 16-bit word, because the

CS8920A does not support 8-bit access to its in-

ternal registers. Figure 5.1 shows the operation

of the ISQ.

The active interrupt pin (INTRQx) is selected via

the Interrupt Number register (PacketPage base +

0370h). As an additional option, all of the inter-

rupt pins can be placed in the high-impedance

state using the same register. See Section 4.3.

An event triggers an interrupt only when the En-

ableIRQ bit of the Bus Control register (bit F of

After the CS8920A has generated an interrupt,

the first read of the ISQ makes the INTRQ out-

put pin go low (inactive). INTRQ remains low

until the null word (0000h) is read from the ISQ,

or for 1.6

µs, whichever is longer.

Typically, when interrupts have been enabled for

various error conditions (runt, CRC error, frame

> 1518 bytes) via register 3, the software will

also select that those frames be discarded (regis-

ter 5).

CS8920A

DS238PP2

CS8920A Datasheet(PDF) 86 Page - Cirrus Logic

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