CS89712 datasheet(60/170 Pages) CIRRUS

60 / 170 page

CS89712

DS502PP2

the RxStatus register, and its length can be read

from the RxLength register.

2.32.6

Receive Frame Byte Counter

The receive frame byte counter describes the num-

ber of bytes received for the current frame. The

counter is incremented in real time as bytes are re-

ceived from the Ethernet. The byte counter can be

used by the driver to determine how many bytes are

available for reading out of the Ethernet port. Max-

imum Ethernet throughput can be achieved by ded-

icating the CPU to reading this counter, and using

the count to read the frame out of the Ethernet port

at the same time it is being received by the

CS89712 from the Ethernet (parallel frame-recep-

tion and frame-read-out tasks).

Following an RxDest or Rx128 interrupt the regis-

ter contains the number of bytes which are avail-

able to be read by the CPU. When the end of frame

is reached, the count contains the final count value

for the frame, including the allowance for the Buff-

erCRC option. When this final count is read by the

CPU the count register is set to zero. Therefore to

read a complete frame using the byte count register,

the register can be read and the data moved until a

count of zero is detected. Then the RxEvent regis-

ter can be read to determine the final frame status.

The sequence is as follows:

1) At the start of a frame, the byte counter matches

the incoming character counter. The byte

counter will have an even value prior to the end

of the frame.

2) At the end of the frame, the final count, includ-

ing the allowance for the CRC (if the Buffer-

CRC option is enabled), is held until the byte

counter is read.

3) When a read of the byte counter returns a count

of zero, the previous count was the final count.

The count may now have an odd value.

4) RxEvent should be read to obtain a final status

of the frame, followed by a Skip command to

complete the operation.

Note that all RxEvents should be processed before

using the byte counter. The byte counter should be

used following a BufEvent when RxDest or Rx128

interrupts are enabled.

2.32.7

Receive Ethernet Port Locations

The receive status/length/frame locations are read

through repetitive reads from one Ethernet port at

the I/O base address.

Random access is not needed. However, the first

118 bytes of the receive frame can be accessed ran-

domly if word reads, on even word boundaries, are

used. Beyond 118 bytes, the memory reads must be

sequential. Byte reads, or reads on odd-word

boundaries, can be performed only in sequential

read mode.

The RxStatus word reports the status of the current

received frame. RxEvent has the same contents as

the RxStatus register, except RxEvent is cleared

when read.

The RxLength (receive length) word is the length,

in bytes, of the data to be transferred to the host

RAM. The register describes the length from the

start of Destination Address to the end of CRC, as-

suming that CRC has been selected (via RxCFG

lected, then the length does not include the CRC,

and the CRC is not present in the receive buffer.

After the RxLength has been read, the receive

frame can be read. When some portion of the frame

is read, the entire frame should be read before read-

ing the RxEvent register either directly or through

the ISQ register. Reading the RxEvent register sig-

nals to the Ethernet port that the software is fin-

ished with the current frame, and wants to start

processing the next frame. In this case, the current

frame will no longer be accessible. The current

frame will also become inaccessible if a Skip com-

CS89712 Datasheet(PDF) 60 Page - Cirrus Logic

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