CIRCC datasheet(57/70 Pages) SMSC

57 / 70 page

writes. This is required both for normal operation

with asynchronous host/SCE access timing, and

during loopback tests with synchronous SCE-

only

access

timing

where

the

FIFO

simultaneously used for transmit and receive.

FIFO controls include, separate read/ write lines,

FIFO Full and FIFO Not Empty flags, Reset,

FIFO Threshold, and Interrupt.

FIFO Threshold

The

SCE

FIFO

Threshold

generates

programmed

I/O

service

requests

accommodate systems with widely varying I/O

response times. FIFO Threshold values typically

reflect the overall I/O response characteristics of

a system.

The same threshold value can be

used for both I/O read and I/O write cases.

During DMA operations, the FIFO Threshold is

only used to trigger the SCE transmitter. NOTE:

the DMA controller will fill the FIFO until the FIFO

Threshold

has

been

exceeded

before

the

transmitter is enabled.

The FIFO Threshold value is programmable from

0 to 31. The FIFO Threshold Register, located in

FIFO Threshold value.

Low threshold values

result in longer periods of time between service

requests because more of the FIFO is utilized

before the request is issued.

Systems that

program low threshold values must typically

provide fast response times to these requests;

i.e., high performance systems that move I/O

data quickly.

High threshold values are used in "sluggish"

systems with long service request latencies. Low

performance systems typically take longer to

move I/O data and require more frequent I/O

service.

For systems that program high FIFO

threshold values, much less of the FIFO is

utilized before service requests are issued.

Receive Threshold

Once the FIFO Interrupt is enabled, Receive

Service

Requests

(RxServReq),

i.e.

data

transfers from the FIFO to the host, are

generated whenever there are 32 minus the

FIFO Threshold value or more data bytes in the

FIFO, given by:

RxServReq

≥ 32 - FIFO Threshold

For example, if the FIFO threshold value is 12,

RxServReq will be active whenever there are 20

to 32 data bytes in the FIFO.

If the FIFO

Threshold

RxServReq

will

active

whenever the FIFO is full. If the FIFO Threshold

is 31, RxServReq will be active whenever the

FIFO is not empty.

Transmit Threshold

Once the FIFO Interrupt is enabled, Transmit

Service

Requests

(TxServReq),

i.e.

data

transfers from the host to the FIFO, are

generated whenever there are FIFO Threshold

value or fewer data bytes in the FIFO, given by:

TxServReq

≤ FIFO Threshold

For example, if the FIFO Threshold value is 12,

TxServReq will be active whenever there are 12

or less data bytes in the FIFO.

If the FIFO

Threshold

TxServReq

will

active

whenever the FIFO is empty. If the FIFO

Threshold is 31, TxServReq will be active

whenever the FIFO is not full.

FIFO Interrupt

The FIFO Interrupt becomes active whenever the

FIFO Interrupt Enable is active and either

TxServReq or RxServReq is active. When FIFO

Interrupt Enable becomes inactive, the FIFO

Interrupt goes inactive.

For example, the FIFO Interrupt will become

active during a transmit operation if the FIFO

Threshold is fifty, the FIFO Interrupt Enable is

active, and there are from one to fifty data bytes

in the FIFO (Figure 22).

In Figure 22, notice that five bytes are written to

the FIFO every time a service request is

answered. The third request occurs as soon as

the FIFO Interrupt Enable is activated because

the five bytes written to the FIFO following the

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