ENC424J600 datasheet(87/168 Pages) MICROCHIP

87 / 168 page

 2010 Microchip Technology Inc.

DS39935C-page 85

ENC424J600/624J600

While transmission is active (TXRTS is set), it is recom-

mended that ETXST and ETXLEN, as well as the

TXMAC bit (ECON2<13>), not be modified. Since

ERXST controls the end of the transmit buffer, and

therefore, buffer wrap-around, it must also remain

unchanged.

To transmit a packet:

Initialize the MAC as described in Section 8.6

“MAC Initialization”

. Most applications should

leave PADCFG<3:0> and TXCRCEN set to their

default values, which enables automatic padding

and CRC generation. For automatic insertion of

the source MAC address during transmission, set

the TXMAC bit to ‘1’.

If desired, enable the transmit done and/or

transmit abort interrupts by setting TXIE and/or

TXABTIE (EIE<3:2>). Clear TXIF and TXABTIF

(EIR<3:2>) if they are currently set. To generate

the interrupt, also set INTIE (EIE<15>).

Copy the packet to the SRAM buffer.

Program ETXST to the start address of the

packet.

Program ETXLEN with the length of data copied

to the memory.

Set the TXRTS bit to initiate transmission.

Wait for the hardware to clear TXRTS and trigger

a transmit interrupt, indicating transmission has

completed.

Read the ETXSTAT register for status information

as described in the next section.

The transmit function does not modify the ETXST

Pointer or ETXLEN data length after the operation

completes. To send another packet, the Start Pointer

must be manually moved to the location of the next

packet and the transmit length must be updated. If

desired, the application can retransmit the last packet

by setting TXRTS again without modifying ETXST or

ETXLEN.

9.1.1

TRANSMISSION STATUS

After transmitting a packet (either successfully or

unsuccessfully), the ETXSTAT and ETXWIRE registers

contain status information about the operation. The

values in these registers will persist until the next

packet is transmitted (again, either successfully or

unsuccessfully). Therefore, ETXSTAT and ETXWIRE

should be treated as valid only when TXRTS is clear.

The

LATECOL

(ETXSTAT<10>),

MAXCOL

(ETXSTAT<9>) and EXDEFER (ETXSTAT<8>) bits are

error flags indicating that packet transmission has

failed. (These errors are possible only in Half-Duplex

mode; therefore, these status bits should be ignored

when operating in Full-Duplex mode.) The device

asserts these flags and clears the TXRTS bit to prevent

a single packet from stalling device operation. When

any of these flags are set, the packet was not success-

fully transmitted and the host controller should

determine whether to retry or ignore the error.

The CRCBAD (EXTSTAT<4>) bit is a warning. It is only

meaningful when automatic CRC generation is dis-

abled and indicates that the checksum computed by

the MAC did not match the one appended by software.

If the software CRC is incorrect, the packet will be

rejected by the remote node. When automatic MAC

hardware generation of the CRC is enabled, this bit can

be ignored as the CRC is always correct.

The DEFER bit (ETXSTAT<7>) and the COLCNT<3:0>

bits (ETXSTAT<3:0>) are status indicators. DEFER

simply indicates that the device had to wait before

transmitting due to flow control or other traffic on the

network. The COLCNT bits indicate the number of

collisions that occurred before the packet was

successfully transmitted.

The ETXWIRE register is a count of the number of

actual bytes the MAC transmitted onto the physical

medium before the transmission completed, either

successfully or unsuccessfully. In Full-Duplex mode,

this count is the total length of the packet, including

padding and CRC. In Half-Duplex mode, this status

due to any collisions that occurred. Therefore, it can be

used to gauge how much total bandwidth the

application is using.

9.1.2

SPECIAL CASE TRANSMISSION

When the value of ETXLEN is 07h or less, the ability to

set the TXRTS bit is locked out in hardware. This is

because the resulting packet would be unable to meet

IEEE 802.3 requirements.

If the PHY is unlinked at the time software sets the

TXRTS bit to transmit a packet, the transmission will

complete normally with applicable interrupts still occur-

ring. However, the PHY submodule will also suppress

the transmission of any data onto the physical medium.

This avoids interference with auto-negotiation, which

may be already using the physical medium. This

behavior is also necessary to meet IEEE 802.3

specifications.

If an attempt is made to transmit a packet that is larger

than specified in the MAC Maximum Frame Length reg-

ister, and huge frames are disabled (MACON2<2> = 0),

the transmission will start normally. However, once the

MAC has transmitted the number of bytes defined in

MAMXFL, the MAC will immediately cease transmis-

sion. This results in the packet being partially transmitted

and then truncated without a valid CRC being

appended. In almost all cases, this results in the remote

node rejecting the packet as having an invalid CRC.

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