ENC28J60-CSLASHSP datasheet(47/96 Pages) MICROCHIP

47 / 96 page

Preliminary

DS39662B-page 45

ENC28J60

7.2.4

FREEING RECEIVE BUFFER SPACE

After the host controller has processed a packet (or part

of the packet) and wishes to free the buffer space used

by the processed data, the host controller must

advance the Receive Buffer Read Pointer, ERXRDPT.

The ENC28J60 will always write up to, but not includ-

ing, the memory pointed to by the Receive Buffer Read

Pointer. If the ENC28J60 ever attempts to overwrite the

Receive Buffer Read Pointer location, the packet in

progress will be aborted, the EIR.RXERIF will be set

and an interrupt will be generated (if enabled). In this

manner,

the

hardware

will

never

overwrite

unprocessed packets. Normally, the ERXRDPT will be

advanced to the value pointed to by the next Packet

Pointer which precedes the receive status vector for

the current packet. Following such a procedure will not

require any Pointer calculations to account for

wrapping at the end of the circular receive buffer.

The

Receive

Buffer

Read

Pointer

Low

Byte

(ERXRDPTL register) is internally buffered to prevent

the Pointer from moving when only one byte is updated

through the SPI. To move ERXRDPT, the host control-

ler must write to ERXRDPTL first. The write will update

the internal buffer but will not affect the register. When

the host controller writes to ERXRDPTH, the internally

buffered low byte will be loaded into the ERXRDPTL

read in any order. When they are read, the actual value

of the registers will be returned. As a result, the

buffered low byte is not readable.

In addition to advancing the Receive Buffer Read

Pointer, after each packet is fully processed, the host

controller must write a ‘1’ to the ECON2.PKTDEC bit.

Doing so will cause the EPKTCNT register to

decrement by 1. After decrementing, if EPKTCNT is ‘0’,

the EIR.PKTIF flag will automatically be cleared.

Otherwise, it will remain set, indicating that additional

packets are in the receive buffer and are waiting to be

processed. Attempts to decrement EPKTCNT below 0

are ignored. Additionally, if the EPKTCNT register ever

maximizes at 255, all new packets which are received

will be aborted, even if buffer space is available.

indicate the error, the EIR.RXERIF will be set and an

interrupt will be generated (if enabled). To prevent this

condition, the host controller must properly decrement

the counter whenever a packet is processed.

Because only one Pointer is available to control buffer

area ownership, the host controller must process pack-

ets in the order they are received. If the host controller

wishes to save a packet to be processed later, it should

copy the packet to an unused location in memory. It

may accomplish this efficiently using the integrated

DMA controller (see Section 13.0 “Direct Memory

Access Controller”).

7.2.5

RECEIVE BUFFER FREE SPACE

At any time the host controller wishes to know how

much receive buffer space is remaining, it should read

the Hardware Write Pointer (ERXWRPT registers) and

compare it with the ERXRDPT registers. Combined

with the known size of the receive buffer, the free space

can be derived.

When reading the ERXWRPT register with the receive

hardware enabled, special care must be taken to

ensure the low and high bytes are read as a matching

set.

To be assured that a matching set is obtained:

Read the EPKTCNT register and save its

contents.

Read ERXWRPTL and ERXWRPTH.

Read the EPKTCNT register again.

Compare the two packet counts. If they are not

the same, go back to step 2.

With the Hardware Write Pointer obtained, the free

space can be calculated as shown in Example 7-2. The

hardware prohibits moving the Write Pointer to the

same value occupied by ERXRDPT (except when the

Buffer Pointers are being configured), so at least one

byte will always go unused in the buffer. The example

calculation reflects the lost byte.

EXAMPLE 7-2:

RECEIVE BUFFER FREE SPACE CALCULATION

Note:

The ERXWRPT registers only update

when a packet has been successfully

received. If the host controller reads it just

before another packet is to be successfully

completed, the value returned could be

stale and off by the maximum frame length

permitted (MAMXFLN) plus 7. Further-

more, as the host controller reads one

byte of ERXWRPT, a new packet may

arrive and update the Pointer before the

host controller has an opportunity to read

the other byte of ERXWRPT.

if ERXWRPT > ERXRDPT, then

Free Space = (ERXND – ERXST) – (ERXWRPT – ERXRDPT)

else if ERXWRPT = ERXRDPT, then

Free Space = (ERXND – ERXST)

else

Free Space = ERXRDPT – ERXWRPT – 1

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