CS8900A datasheet(71/128 Pages) CIRRUS

71 / 128 page

DS271PP3

CS8900A

Crystal LAN™ ISA Ethernet Controller

CIRRUS LOGIC PRODUCT DATA SHEET

4.5 Initiate Transmit Registers

4.5.1 Transmit Command Request - TxCMD

(Write-only, Address: PacketPage base + 0144h)

The word written to PacketPage base + 0144h tells the CS8900A how the next packet should be transmitted. This

PacketPage location is write-only, and the written word can be read from Register 9, at PacketPage base + 0108h.

The CS8900A does not transmit a frame if TxLength (at PacketPage location base + 0146h) is less than 3. See

Section 5.7 on page 99.

001001

These bits provide an internal address used by the CS8900A to identify this as the Transmit

Command Register. When reading this register, these bits will be 001001, where the LSB cor-

responds to Bit 0.

TxStart

This pair of bits determines how many bytes are transferred to the CS8900A before the MAC

starts the packet transmit process.

Bit 7

Bit 6

Start transmission after 5 bytes are in the CS8900A

Start transmission after 381 bytes are in the CS8900A

Start transmission after 1021 bytes are in the CS8900A

Start transmission after the entire frame is in the CS8900A

Force

When set in conjunction with a new transmit command, any transmit frames waiting in the trans-

mit buffer are deleted. If a previous packet has started transmission, that packet is terminated

within 64 bit times with a bad CRC.

Onecoll

When this bit is set, any transmission will be terminated after only one collision. When clear, the

CS8900A allows up to 16 normal collisions before terminating the transmission.

InhibitCRC

When set, the CRC is not appended to the transmission.

TxPadDis

When TxPadDis is clear, if the host gives a transmit length less than 60 bytes and InhibitCRC

is set, then the CS8900A pads to 60 bytes. If the host gives a transmit length less than 60 bytes

and InhibitCRC is clear, then the CS8900A pads to 60 bytes and appends the CRC.

When TxPadDis is set, the CS8900A allows the transmission of runt frames (a frame less than

64 bytes). If InhibitCRC is clear, the CS8900A appends the CRC. If InhibitCRC is set, the

CS8900A does not append the CRC.

Since this register is write-only, it’s initial state after reset is undefined.

4.5.2 Transmit Length

(Write-only, Address: PacketPage base + 0146h)

This register is used in conjunction with register 9, TxCMD. When a transmission is initiated via a command in Tx-

CMD, the the length of the transmitted frame is written into this register. The length of the transmitted frame may be

modified by the configuration of the TxPadDis and InhibitCRC bits in the TxCMD register. See Table 35, and

Section 5.7 on page 99. TxLength must be >3 and < 1519.

Since this register is write-only, it’s initial state after reset is undefined.

765

4321

TxStart

001001

DCBA

TxPadDis

InhibitCRC

Onecoll

Force

Address 0147h

Address 0146h

Most-significant byte of Transmit Frame Length

Least-significant byte of Transmit Frame Length

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