AM79C90 Advanced Micro Devices, AM79C90 Datasheet - Page 30

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

Transmit Message Descriptor 3 (TMD3)
Bit
15
14
13
12
11
30
15
AMD
Name
BUFF
UFLO
LCOL
LCAR
RES
10 9
Description
BUFFER ERROR is set by the
C-LANCE during transmission when
the C-LANCE does not find the ENP
flag in the current buffer and does
not own the next buffer. This can oc-
cur in either of two ways: either the
OWN bit of the next buffer is zero, or
Transmit FIFO underflow occurred
before the C-LANCE has performed
a lookahead poll of the next transmit
descriptor. BUFF is set by the
C-LANCE and cleared by the host.
BUFF error will turn off the transmit-
ter (CSR
If a Buffer Error occurs, an Underflow
Error will also occur. BUFF error is
not valid when LCOL or RTRY error
is set during TX data chaining.
UNDERFLOW ERROR indicates
that the transmitter has truncated a
message due to data late from mem-
ory. UFLO indicates that the Trans-
mit FIFO has emptied before the end
of the packet was reached.
Upon UFLO error, transmitter is
turned off (CSR
RESERVED bit. The C-LANCE will
write this bit with a “0.”
LATE COLLISION indicates that a
collision has occurred after the slot
time of the channel has elapsed. The
C-LANCE does not retry on late
collisions.
LOSS OF CARRIER is set when the
carrier
C-LANCE goes false during a
C-LANCE-initiated
The C-LANCE does not retry upon
loss of carrier. It will continue to
transmit the whole packet until done.
LCAR is not valid in INTERNAL
LOOPBACK MODE.
0
input
, TXON = 0).
0
, TXON = 0).
(RENA)
transmission.
0
17881B-32
P R E L I M I N A R Y
LCAR
LCOL
TDR
RTRY
RES
UFLO
BUFF
to
the
Am79C90
10
09:00
Ring Access Mechanism in the C-LANCE
Once the C-LANCE is initialized through the initializa-
tion block and started, the CPU and the C-LANCE com-
municate via transmit and receive rings, for packet
transmission and reception.
There are 2 sets of RAM locations (four 16-bit register
per set, corresponding to the 4 entries in each descrip-
tor) in the C-LANCE. The first set points to the current
buffer, and they are the working registers which are
used for transferring the data for the packet. The second
set contains the pointers to the next buffer in the ring
which the C-LANCE obtained from the lookahead
operation.
There are three types of ring access in the C-LANCE.
The first type is when the C-LANCE polls the rings to
own a buffer. The second type is when the buffers are
data chained. The C-LANCE does a lookahead opera-
tion between the time that it is transferring data to/from
the Transmit/Receive FIFOs; this lookahead is done
only once. The third type is when the C-LANCE tries to
own the next descriptor in the ring when it clears the
OWN bit for the current buffer.
Transmit Ring Buffer Management
When there is no Ethernet activity, the C-LANCE will
automatically poll the transmit ring in the memory once it
has started (CSR0, STRT = 1). This polling occurs every
1.6 ms, (CSR0 TDMD bit = 0) and consists of reading
the status word of the transmit descriptor, TMD1, until
the C-LANCE owns the descriptor. The C-LANCE will
read TMD0 and TMD2 to get the rest of the buffer ad-
dress and the buffer byte count when it owns the de-
scriptor. Each of these memory reads is done
separately with a new arbitration cycle for each transfer.
If the transmit buffers are data chained (current buffer
ENP = 0), the C-LANCE will look ahead to the next de-
scriptor in the ring while transferring the current buffer
into the Transmit FIFO (see Figure 8-1). The C-LANCE
does this lookahead only once. If it does not own the
next transmit Descriptor Table Entry (DTE) (2nd TX ring
RTRY
TDR
RETRY ERROR indicates that the
transmitter has failed in 16 attempts
to successfully transmit a message
due to repeated collisions on the me-
dium. If DRTY = 1 in the MODE reg-
ister, RTRY will set after 1 failed
transmission attempt.
TIME DOMAIN REFLECTOMETRY
reflects the state of an internal C-
LANCE counter that counts from the
start of a transmission to the occur-
rence of a collision. This value is
useful in determining the approxi-
mate distance to a cable fault. The
TDR
C-LANCE and is valid only if RTRY
is set.
value
is
written
by
the