Datasheet

 2006-2012 Microchip Technology Inc. . DS39662E-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 including, 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. Follow-
ing 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
register at the same time. The ERXRDPT bytes can be
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. To
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:
1. Read the EPKTCNT register and save its
contents.
2. Read ERXWRPTL and ERXWRPTH.
3. Read the EPKTCNT register again.
4. 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 success-
fully completed, the value returned could
be stale and off by the maximum frame
length permitted (MAMXFL) plus 7. Fur-
thermore, 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