AM79C90 Advanced Micro Devices, AM79C90 Datasheet - Page 31

• Manufacturer Part Number: AM79C90
• Description: CMOS Local Area Network Controller for Ethernet (C-LANCE)
• Manufacturer: Advanced Micro Devices

for this packet) it will transmit the current buffer and up-
date the status of current Ring with the BUFF and UFLO
error bits set. If the C-LANCE owns the 2nd DTE, it will
also read the buffer address and the buffer byte count of
this entry. Once the C-LANCE has finished emptying the
current buffer, it clears the OWN bit for this buffer, and
immediately starts loading the Transmit FIFO from the
next (2nd) buffer. Between DMA bursts, starting from
the 2nd buffer, the C-LANCE does a lookahead again to
check if it owns the next (3rd) buffer. This activity goes
on until the last transmit DTE indicates the end of the
packet (TMD1, ENP = 1). Once the last part of the pack-
et has been transmitted out from the Transmit FIFO to
the medium, the C-LANCE will update the status in
TMD1, TMD3 (TMD3 is updated only when there is an
error) and will relinquish the last buffer to the CPU. The
C-LANCE tries to own the next buffer (first buffer of the
next packet), immediately after it relinquishes the last
buffer of the current packet. This guarantees the back-
to-back transmission of the packets. If the C-LANCE
does not own the next buffer, it then polls the TX ring
every 1.6 ms.
When an error occurs before all of the buffers get trans-
mitted, the status, TMD3 , is updated in the current DTE,
own bit is cleared in TMD1, and TINT bit is set in CSR0
which causes an interrupt if INEA = 1. The C-LANCE will
then skip over the rest of the descriptors for this packet
(clears the OWN bit and sets the TINT bit in CSR0) until
it finds a buffer with both the STP and OWN bit being set
(this indicates the first buffer for the next packet).
When the transmit buffers are not data chained (current
descriptor’s ENP = 1), the C-LANCE will not perform any
lookahead operation. It will transmit the current buffer,
update the TMD3 if any error, and then update the
status and clear the OWN bit in TMD1 . The C-LANCE
will then immediately check the next descriptor in the
ring to see if it owns it. If it does, the C-LANCE will also
read the rest of the entries from the descriptor table. If
the C-LANCE does not own it, it will poll the ring once
every 1.6 ms until it owns it. User may set the TDMD bit
in CSR0 when it has relinquished a buffer to the
C-LANCE. This will force the C-LANCE to check the
OWN bit at this buffer without waiting for the polling time
to elapse.
Receive Ring Buffer Management
Receive Ring access is similar to the transmit ring ac-
cess. Once the receiver is enabled, the C-LANCE will al-
ways try to have a receive buffer available, should there
be a packet addressed to this node for reception. There-
fore, when the C-LANCE is idle, it will poll the receive
ring entry once every 1.6 ms, until it owns the current re-
ceive DTE. Once the C-LANCE owns the buffer, it will
read RMD0 and RMD2 to get the rest of buffer address
and buffer byte count. When a packet arrives from the
physical medium, after the Address Recognition Logic
accepts the packet, the C-LANCE will immediately poll
P R E L I M I N A R Y
Am79C90
the Receiver Ring once for a buffer. If it still does not own
the buffer, it will set the MISS error in CSR0 and will not
poll the receive ring until the packet ends.
Assuming the C-LANCE owns a receive buffer when the
packet arrives, it will perform a lookahead operation on
the next DTE between periods when it is dumping the re-
ceived data from the Receive FIFO to the first receive
buffer in case the current buffer requires data chaining.
When the C-LANCE owns the buffer, the lookahead op-
eration consists of three separate single word DMA
reads: RMD1, RMD0, and RMD2. When the C-LANCE
does not own the next buffer, the lookahead operation
consists of only one single DMA read, RMD1. Either
lookahead operation is done only once. Following the
lookahead operation, whether C-LANCE owns the next
buffer or not, the C-LANCE will transfer the data from
Receive FIFO to the first receive buffer for this packet in
burst mode (8 word transfer per one DMA cycle
arbitration).
If the packet being received requires data chaining, and
the C-LANCE does not own the second DTE, the
C-LANCE will update the current buffer status, RMD1,
with the BUFF and/or OFLO error bits set. If the
C-LANCE does own the next buffer (second DTE) from
previous lookahead, the C-LANCE will relinquish the
current buffer and start filling up the second buffer for
this packet. Between the time that the C-LANCE is
transferring data from the Receive FIFO to the second
buffer, it does a lookahead operation again to see if it
owns the next (third) buffer. If the C-LANCE does own
the third DTE, it will also read RMD0, and RMD2 to get
the rest of buffer pointer address and buffer byte count.
This activity continues on until the C-LANCE recognizes
the end of the packet (physical medium is idle); it then
updates the current buffer status with the end of packet
bit (ENP) set. The C-LANCE will also update the mes-
sage byte count (RMD3) with the total number of bytes
received for this packet in the current buffer (the last
buffer for this packet).
The dual FIFOs in the C-LANCE are utilized by the inter-
nal microcode to guarantee that continuous receive ac-
tivity does not prevent the servicing of pending transmit
packets. The microcode includes a single transmit de-
scriptor poll operation at the beginning of buffer DMA
operations for an incoming receive packet. This single
transmit descriptor poll is performed only once during
the receive microcode routine for each packet that is re-
ceived. If the OWN bit in the transmit descriptor is set,
burst transfers to the Transmit FIFO are interleaved with
burst transfers from the Receive FIFO. By interleaving
the transmit buffer transfers with the receive buffer
transfers, the beginning of the transmit packet is
preloaded in the Transmit FIFO, ready to be transmitted
immediately following the end of the receive packet on
the wire.
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