MA8251
CPU BYTE (5-8 BITS/CHARACTER)
DATA CHARACTER
ASSEMBLED SERIAL DATA OUTPUT (TxD)
START BIT
DATA CHARACTER
PARITY BIT
STOP BITS
Figure 6: Transmission Format
SERIAL DATA INPUT (RxD)
START BIT
DATA CHARACTER
PARITY BIT
STOP BITS
CPU BYTE (5-8 BITS/CHARACTER) (See Note Below)
DATA CHARACTER
NOTE: If character length is defined as 5, 6 or 7 bits the unused bits are set to zero.
Figure 7: Receive Format
3.6 SYNCHRONOUS MODE (TRANSMISSION)
synchronization. The SYNDET pin is then set high, and is reset
automatically by a STATUS READ. If parity is programmed,
SYNDET will not be set until the middle of the parity bit, instead
of the middle of the last data bit.
In the external SYNC mode, synchronization is achieved
by applying a high level on the SYNDET pin, thus forcing the
MA8251 out of the HUNT mode. The high level can be
removed after one RxC cycle. An ENTER HUNT command
has no effect in the asynchronous mode of operation.
The TxD output is continuously high until the CPU sends its
first character to the MA8251 which usually is a SYNC
character. When the CTS line goes low, the first character is
serially transmitted out. All characters are shifted out on the
falling edge of TxC. Data is shifted out at the same rate as the
TxC.
Once transmission has started, the data stream at the TxD
output must continue at the TxC rate. If the CPU does not
provide the MA8251 with a data character before the MA8251
Transmitter Buffers become empty, the SYNC characters (or
character if in single SYNC character mode) will be
automatically inserted in the TxD data stream. In this case, the
TxEMPTY does not go low when the SYNC is being shifted out
(see figure 8). The TxEMPTY pin is internally reset by a data
character being written into the MA8251.
3.7 SYNCHRONOUS MODE (RECEIVER)
In this mode character synchronisation can be internally or
externally achieved. If the SYNC mode has been
programmed, ENTER-HUNT command should be included in
the first command instruction word written. Data on the RxD
pin is then sampled on the rising edge of RxC. The content of
the Rx buffer is compared to every bit boundary with the first
SYNC character until a match occurs.
If the MA8251 has been programmed for two SYNC
characters, the subsequent received character is also
compared; when both SYNC characters have been detected,
the USART ends the HUNT mode and is in character
Figure 8: Sync Character Insertion
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