CS8900A
Crystal LAN™ ISA Ethernet Controller
5.0 OPERATION
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 always read the
entire 16-bit word.
5.1 Managing Interrupts and Servicing the
Interrupt Status Queue
The Interrupt Status Queue (ISQ) is used by the
CS8900A to communicate Event reports to the host
processor. Whenever an event occurs that triggers The active interrupt pin (INTRQx) is selected via
an enabled interrupt, the CS8900A sets the appro- the Interrupt Number register (PacketPage base +
priate bit(s) in one of five registers, maps the con- 22h). As an additional option, all of the interrupt
tents of that register to the ISQ, and drives the
selected interrupt request pin high (if an earlier in-
terrupt 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 inter-
rupt (the first read to the ISQ causes the interrupt
request pin to go low.)
pins can be 3-Stated using the same register. see
Section 4.3 on page 42.
An event triggers an interrupt only when the En-
ableIRQ bit of the Bus Control register (bit F of
register 17) is set. After the CS8900A has generat-
ed an interrupt, the first read of the ISQ makes the
INTRQ output pin go low (inactive). INTRQ re-
mains low until the null word (0000h) is read from
the ISQ, or for 1.6us, whichever is longer.
Three of the registers mapped to the ISQ are event
registers: RxEvent (Register 4), TxEvent (Register
8), and BufEvent (Register C). The other two reg-
isters are counter-overflow reports: RxMISS (Reg-
ister 10) and TxCOL (Register 12). There may be
more than one RxEvent report and/or more than
5.2 Basic Receive Operation
5.2.0.1 Overview
Once an incoming packet has passed through the
analog front end and Manchester decoder, it goes
one TxEvent report in the ISQ at a time. However, through the following three-step receive process:
there may be only one BufEvent report, one Rx-
MISS report and one TxCOL report in the ISQ at a
time.
1) Pre-Processing
2) Temporary Buffering
3) Transfer to Host
Event reports stored in the ISQ are read out in the
order of priority, with RxEvent first, followed by Figure 20 shows the steps in frame reception.
TxEvent, BufEvent, RxMiss, and then TxCOL.
As shown in the figure, all receive frames go
The host only needs to read from one location to get
through the same pre-processing and temporary
the interrupt currently at the front of the queue. In
buffering phases, regardless of transfer method
Memory Mode, the ISQ is located at PacketPage
Once a frame has been pre-processed and buffered,
base + 0120h. In I/O Mode, it is located at I/O base
it can be accessed by the host in either Memory or
+ 0008h. Each time the host reads the ISQ, the bits
I/O space. In addition, the CS8900A can transfer
in the corresponding register are cleared and the
receive frames to host memory via host DMA. This
next report in the queue moves to the front.
section describes receive frame pre-processing and
When the host starts reading the ISQ, it must read
Memory and I/O space receive operation.
and process all Event reports in the queue. A read-
Section 5.4 on page 90 through Section 5.5 on
out of a null word (0000h) indicates that all inter-
page 93 describe DMA operation.
rupts have been read.
CIRRUS LOGIC PRODUCT DATA SHEET
DS271PP3
79
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