AM8530H AMD [Advanced Micro Devices], AM8530H Datasheet - Page 80

• Manufacturer Part Number: AM8530H
• Description: Serial Communications Controller
• Manufacturer: AMD [Advanced Micro Devices]

AMD
(so the Reset Tx CRC Generator command is also not necessary), and the Tx Underrun/
EOM latch will be reset automatically on every new frame sent. This ensures that an
opening flag and proper CRC generation and transmission will always be sent without
processor intervention under varying bus latency conditions.
4.8.4
The premature termination of a frame is called an “abort”. A properly transmitted SDLC
frame will be terminated by appending the CRC characters and a closing flag, but the
SCC may be programmed to terminate the frame by sending an abort and a flag instead.
This option allows the SCC to abort the transmission of a frame in progress and at the
same time signify to the receiver that another frame will follow.
This is controlled by the ABORT/FLAG on Underrun bit in WR10 (D2). When this bit is
set to ‘1’, and an underrun occurs, the transmitter will transmit an abort immediately fol-
lowed by a flag instead of the normal CRC. If this bit is set to ‘0’, the frame will be termi-
nated normally.
The processor is also able to send an abort by issuing the Send Abort command via
WR0. This command, when issued, will send eight consecutive ‘1’s. After this pattern is
transmitted, the transmitter will idle as programmed via bit D3 of WR10. Since up to five
consecutive ‘1’s may have been sent prior to the command being issued, a Send Abort
may cause a sequence of from eight to thirteen ‘1’s to be transmitted. The Send Abort
command also empties the transmit buffer register and sets the Tx Underrun/EOM bit in
RR0.
4.8.5
The NMOS SCC does not automatically preset the CRC generator prior to frame trans-
mission. This must be done in software, usually during the initialization routine. This is
accomplished by issuing the Reset Tx CRC Generator Command via WR0. For proper
results, this command must be issued while the transmitter is enabled and idling and be-
fore any data are written to the Transmit Buffer.
In addition, if CRC is to be used, the transmit CRC generator must be enabled by setting
bit D0 of WR5 to ‘1’. CRC is normally calculated on all characters between opening and
closing flags, so this bit should be set to ‘1’ at initialization and never changed. Note that
a Channel Reset will not initialize the CRC generator so a Reset Tx CRC Generator com-
mand must be issued some time after a Channel Reset is executed.
On the CMOS SCC, setting bit D0 of WR15 ‘1’ will cause the transmit CRC generator to
be preset automatically every time an opening flag is sent, so the Reset Tx CRC Genera-
tor command is not necessary.
4.8.6
The transmission of the CRC check characters is controlled by the Transmit CRC Enable
bit in WR5 (D0) and the Tx Underrun/EOM bit in RR0 (D6). However, if the Transmit CRC
Enable bit is set to ‘0’ when a transmit underrun (i.e., both the Transmit Buffer and Trans-
mit Shift Register go empty) occurs, the CRC check characters will not be sent regardless
of the state of the Tx Underrun/EOM bit.
If the Transmit CRC Enable bit is set to ‘1’ when an underrun occurs, then the state of the
Tx Underrun/EOM bit and the Abort/Flag on Underrun bit in WR10 (D2) determine the
action taken by the transmitter. The Abort/Flag on Underrun bit may be set or reset by the
processor, whereas, the Tx Underrun/EOM bit is set by the transmitter and can be reset
only by the processor via the Reset Tx Underrun/EOM command in WR0.
If the Tx Underrun/EOM bit is set to ‘1’ when an underrun occurs, the transmitter will
close the frame by sending a flag; however, if this bit is set to ‘0’, the frame data will be
appended with either the accumulated CRC characters followed by a flag or an abort pat-
tern followed by a flag, depending on the state of the Abort/Flag on Underrun bit in the
WR10 (D2). In either case, after the closing flag is sent, the Transmitter will idle the trans-
mission line as specified by the Mark/Flag Idle bit D3 in WR10.
4–28
Abort Generation
Auto Transmit CRC Generator Preset
CRC Transmission
Data Communication Modes Functional Description