7.2.4FREEING 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. 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 controller 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 packets 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”).

7.2.5RECEIVE 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.

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 (MAMXFL) plus 7. Furthermore, 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.

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.