AM8530H AMD [Advanced Micro Devices], AM8530H Datasheet - Page 65

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

Data Communication Modes Functional Description
4.6.1.3
A break condition is recognized when a null character (all ‘0’s) plus a Framing Error is
detected by the receiver. Upon recognizing this sequence, the BREAK/ABORT status bit
in RR0 will be set and remains set until a ‘1’ is received indicating that a break condition
is no longer present. Note that at the termination of a break, the Receive Data FIFO con-
tains a single null character, which should be read and discarded. The Framing Error bit
will not be set for this null character, but if odd parity has been selected, the Parity Error
bit will be set. Caution should be exercised if the receive data line contains a switch that
is not debounced to generate breaks. Switch bounce may cause multiple breaks, recog-
nized by the receiver to be additional characters assembled in the Receive Data FIFO. It
may also cause a Receiver Overrun condition to be latched.
4.6.1.4
When an SCC channel is programmed in Asynchronous mode it may be programmed to
accept a transmit/receive clock that is 1, 16, 32, or 64 times the data rate. This is selected
by bits D7 and D6 in WR4. The clock factor chosen will be common to both the transmit-
ter and receiver.
The x1 mode in Asynchronous mode is a combination of both synchronous and asynchro-
nous transmission. The data are clocked by a common timing base, but characters are
still framed with Start and Stop bits. Because the receiver waits for one clock period after
detecting the first High-to-Low transition before beginning to assemble characters, the
data and clock must be synchronized externally. The x1 mode is the only mode in which a
data encoding method other than NRZ may be used.
In SDLC and Synchronous modes bits D7 and D6 of WR4 are ignored because the x1
clock is forced internally.
4.6.2
In Asynchronous mode, WR6 and WR7 are not used and the Transmit Shift Register is
formatted with Start and Stop bits before data are shifted out to the transmit multiplexer at
the selected clock rate. Asynchronous data leaves the Transmit Shift Register and goes
directly to the Transmit Multiplexer. CRC generation is not supported in this mode.
4.6.2.1
The initialization sequence for the transmitter in Asynchronous mode is: WR4 first to se-
lect the mode, then WR3 and WR5 to select the various options. At this point the other
registers should be initialized as necessary. When all of this is complete, the transmitter
may be enabled by setting bit D3 of WR5 to ‘1’.
At this point, the transmitter is enabled and the TxD pin will remain in the marking (High)
state. When the first character is written to WR8, it is transferred to the Transmit Shift
Register and the Transmit Buffer Empty bit is set to ‘1’. A Parity bit (if enabled), Start-bit,
and the selected number of Stop bits are then appended to the character. After the char-
acter has been completely sent, the next character is transferred to the Transmit Shift
Register and the process continues. When no more characters are to be transmitted (i.e.,
the transmitter is completely empty), the All Sent status bit in RR1 (D0) will be set when
the last Stop bit reaches the TxD pin. This bit can be used by the processor as an indica-
tion that the transmitter may be safely disabled. The TxD pin then remains in the marking
state until the next character is written to WR8.
4.6.2.2
The SCC provides three Stop-bit options via bits D3 and D2 in WR4. The options avail-
able are one, one-and-a-half, or two stop bits per character. These two bits in WR4 select
only the number of Stop bits for the transmitter, as the receiver always checks for one
Stop bit. Note that the selected clock factor may restrict the number of Stop bits that may
be transmitted. In particular, when the clock rate and data rate are the same (i.e., x1
mode), one-and-a-half Stop bits are not allowed. If any length other than one Stop bit is
desired in the x1 mode, only two Stop bits can be used.
Break Detection
Clock Selection
Transmitter Operation
Transmitter Initialization
Stop Bit Selection
AMD
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