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As in Asynchronous transmission, the TxD output remains "h igh " (marking)
until the juPD8251 receives the first character from the processor which is usually
a SYNC character. After a Command Instruction has set TxEN and after Clear
to Send (CTS) goes low, the first character is serially transmitted. Data is
shifted out on the falling edge o f TxC and the same rate as TxC.
Once transmission has started. Synchronous Mode form at requires that the serial
data stream at TxD continue at the TxC rate or SYNC w ill be lost. If a data character
is not provided by the processor before the /iPD8251 Transmitter Buffer becomes
empty, the SYNC character(s) loaded directly follow ing the Mode Instruction w ill
be automatically inserted in the TxD data stream. The SYNC character(s) are inserted
to fill the line and maintain synchronization until new data characters are available
for transmission. If the /UPD8251 becomes empty, and must send the SYNC character(s),
the TxEMPTY output is raised to signal the processor that the Transmitter Buffer
is empty and SYNC characters are being transmitted. TxEMPTY is automatically
reset by the next character from the processor.
TxEMPTY go high at the middle of the last data b it when the Transmit Register is
EMPTY. TxEMPTY goes low again as sync characters are transmitted. See figure below.
FOR SINGLE SYNC CHARACTER OPERATION
X
TxD
>( DATA ) (
d a t a
DATA WRITE
TxEMPTY
---------------------------- f?------------------
FOR DOUBLE SYNC CHARACTER OPERATION (BISYNC)
X
TxD
DATA
DATA WRITE
—
rp---------
TxEMPTY
CENTER OF LAST BIT
X
TxD
DATA
_______ r i _ n _ _ n __
DATA WRITE
----r p — i p -----------
TxEMPTY
CENTER OF LAST BIT
In Synchronous Receive, character synchronization can be either external or internal.
If the internal SYNC mode has been selected, and the Enter HUNT (EH) bit
has been set by a Command Instruction, the receiver goes into the HUNT mode.
Incoming data on the RxD input is sampled on the rising edge of RxC, and the
Receiver Buffer is compared w ith the first SYNC character after each b it has been
loaded until a match is found. If two SYNC characters have been programmed, the
next received character is also compared. When the SYNC character(s) programmed
have been detected, the ;tPD8251 leaves the HUNT mode and is in character synchro
nization. A t this time, the SYNDET (output) is set high. SYNDET is automatically
reset by a STATUS READ.
If external SYNC has been specified in the Mode Instruction, a "o n e " applied
to the SYNDET (input) for at least one RxC cycle w ill synchronize the USART.
Parity and Overrun Errors are treated the same in the Synchronous as in the
Asynchronous Mode. Framing errors do not apply in the Synchronous format.
The processor may command the receiver to enter the HUNT mode w ith a Command
Instruction which sets Enter HUNT (EH) if synchronization is lost.
(
SYNC )
SYNC
CENTER OF LAST BIT
)
(
SYNC1 X
SYNC2
'
DATA > (
_ n _ _ _ _ _ _ n _ _ n __
) < [
X
X
DATA
SYNC1
SYNC2
) (
^ ( DATA X
DATA
n_n_n
DATA X
'
X ^ ~
data
SYNCHRONOUS
TRANSMISSION
SYNCHRONOUS
RECEIVE
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