ENC28J60_12 datasheet(39/102 Pages) MICROCHIP

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 2006-2012 Microchip Technology Inc.

DS39662E-page 39

ENC28J60

7.0

TRANSMITTING AND

RECEIVING PACKETS

7.1

Transmitting Packets

The MAC inside the ENC28J60 will automatically gener-

ate the preamble and Start-of-Frame delimiter fields

when transmitting. Additionally, the MAC can generate

any padding (if needed) and the CRC if configured to do

so. The host controller must generate and write all other

frame fields into the buffer memory for transmission.

Additionally, the ENC28J60 requires a single per packet

control byte to precede the packet for transmission. The

per packet control byte is organized as shown in

Figure 7-1. Before transmitting packets, the MAC

registers which alter the transmission characteristics

should be initialized as documented in Section 6.0

“Initialization”.

For an example of how the entire transmit packet and

results will look in memory, see Figure 7-2.

FIGURE 7-1:

FORMAT FOR PER PACKET CONTROL BYTES

—

PHUGEEN

PPADEN

PCRCEN

POVERRIDE

bit 7

bit 0

bit 7-4

Unused

bit 3

PHUGEEN: Per Packet Huge Frame Enable bit

When POVERRIDE = 1:

1 = The packet will be transmitted in whole

0 = The MAC will transmit up to the number of bytes specified by MAMXFL; if the packet is larger

than MAMXFL, it will be aborted after MAMXFL is reached

When POVERRIDE = 0:

This bit is ignored.

bit 2

PPADEN: Per Packet Padding Enable bit

When POVERRIDE = 1:

1 = The packet will be zero-padded to 60 bytes if it is less than 60 bytes

0 = The packet will be transmitted without adding any padding bytes

When POVERRIDE = 0:

This bit is ignored.

bit 1

PCRCEN: Per Packet CRC Enable bit

When POVERRIDE = 1:

1 = A valid CRC will be calculated and attached to the frame

0 = No CRC will be appended; the last 4 bytes of the frame will be checked for validity as a CRC.

When POVERRIDE = 0:

This bit is ignored.

bit 0

POVERRIDE: Per Packet Override bit

1 = The values of PCRCEN, PPADEN and PHUGEEN will override the configuration defined by

MACON3

0 = The values in MACON3 will be used to determine how the packet will be transmitted

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