Cirrus Logic CS8900A - Manual (Page 69)

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CS8900A
Crystal LAN™ Ethernet Controller
CIRRUS LOGIC PRODUCT DATASHEET
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 5.6 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
0 0 Start transmission after 5 bytes are in the CS8900A
0 1 Start transmission after 381 bytes are in the CS8900A
1 0 Start transmission after 1021 bytes are in the CS8900A
1 1 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-
7 6 5 4 3 2 1 0
TxStart 001001
F E D C B A 9 8
TxPadDis InhibitCRC Onecoll Force
Address 0147h Address 0146h
Most-significant byte of Transmit Frame Length Least-significant byte of Transmit Frame Length