Am8251 • Am9551
INTERFACE SIGNAL DESCRIPTION
The Am9551 uses an 8 bit bi-directional data bus to exchange
information with an associated processor. Internally, data is
routed between the data bus buffers and the transmitter sec-
tion or receiver section as selected by the Read (RD) or Write
(WR) control inputs.
Chip Select (CS)
The active low Chip Select input allows the Am9551 to be in-
dividually selected from other devices within its address range.
When Chip Select is high, reading or writing is inhibited, and
the data bus output is in it's high impedance state.
The Am9551 will assume an idle state when a high level is
applied to the Reset input. When the Reset is returned low,
the Am9551 will remain in the idle state until it receives a new
mode control instruction.
The active low Read input enables data to be transferred from
the Am9551 to the processor.
The active low Write input enables data to be transferred from
the processor to the Am9551.
During a Read operation, if this input is at a high level the
status byte will be read, and if it is at a low level the receive
data will be read by the processor. When a Write operation is
being performed, this input will indicate to the Am9551 that
the bus information being written is a command if ci5 is high
and data if cio is low.
C/O RD WR CS
0 0 1 0 Am9551 DATA ~ DATA BUS
0 1 0 0 DATA BUS~ Am9551 DATA
1 0 Am9551 STATUS~ DATA BUS
1 1 0 0 DATA BUS~Am9551 COMMAND
X X X 1 DATA BUS ~ 3-STATE
This input is used for internal timing within the Am9551. It
does not control the transmit or receive rate. However, it
should be at least 30 times the receive or transmit rate in the
synchronous mode and 4.5 times the receive or transmit rate
in the asynchronous mode. The ClK frequency is also re-
stricted by both an upper and a lower bound. This input is
often connected to a clock from the associated processor.
Receiver Data (RxD)
Serial data is received from the communication line on this
Receiver Clock (RxC)
The serial data on input RxD is clocked into the Am9551 by
the RxC clock signal. In the synchronous mode, RxC is deter-
mined by the baud rate and supplied by the modem. In the
asynchronous mode, RxC is 1, 16, or 64 times the baud rate as
selected in the mode control instruction. Data is sampled by
the Am9551 on the rising edge of RxC.
Receiver Ready (RxRDY)
The RxRDY output signal indicates to the processor that data
has been shifted into the receiver buffer from the receiver sec-
tion and may be read. The signal is active high and will be re-
set when the buffer is read by the processor. RxRDY can be
activated only if the receiver enable (RxE) has been set in the
command register, even though the receiver may be running.
If the processor does not read the receiver buffer before the
next character is shifted from the receiver section then an
overrun error will be indicated in the status buffer.
Sync Detect (SYNDET)
This signal is used only in the synchronous mode. It can be
either an output or input depending on whether the program
is set for internal or external synchronization. As an output,
a high level indicates when the sync character has been de-
tected in the received data stream after the Internal Synchron-
ization mode has been programmed. If the Am9551 is pro-
grammed to utilize two sync characters, then SYNDET will go
to a high level when the last bit of the second sync character
is received. SYNDET is reset when the status buffer is read or
when a Reset signal is activated. SYNDET will perform as an
input when the External Synchronization mode is programmed.
External logic can supply a positive-going signal to indicate to
the Am9551 that synchronization has been attained. This will
cause it to initialize the assembly of characters on the next
falling edge of RxC. To successfully achieve synchronization
the SYNDET signal should be maintained in a high condition
for at least one full period of RxC.
Transmit Data (TxD)
Serial data is transmitted to the communication line on this
Transmitter Clock (TxC)
The serial data on TxD is clocked out with the TxC signal. The
relationship between clock rate and baud rate is similar to that
for RxC. Data is shifted out of the Am9551 on the falling edge
Transmitter Ready (TxRDY)
The TxRDY output signal goes high when data in the Transmit
Data Buffer has been shifted into the transmitter section al-
lowing the Transmit Data Buffer to accept the next byte from
the processor. TxRDY will be reset when information is writ-
ten into the Transmit Data Buffer. loading command register
also resets TxRDY. TxRDY will be available on this output
pin only when the Am9551 is enable to transmit (CTS = 0,
TxEN = 1). However, the TxRDY bit in the status Buffer will
always be set when the Transmit Data Buffer is empty regard-
less of the state of TxEN and CTS.
Transmitter Empty (TxE)
The TxE output signal goes high when the Transmitter section
has transmitted its data and is empty. The signal will remain
high until a new data byte is shifted from the Transmit Data
Buffer to the Transmitter section. In the synchronous mode if
the processor does not load a new byte into the buffer in time,
TxE will, independent of the status of the TxEN bit in the
command register, momentarily go to a high level as SYNC
characters are loaded into the Transmitter Section.
Data Terminal Ready (DTR)
This signal is a general purpose output which reflects the state
of bit 1 in the Command instruction. It is commonly connected
to an associated modem to indicate that the Am9551 is ready.
Data Set Ready (DSR)
This is a general purpose input signal and forms part of the sta-
tus byte that may be read by the processor. DSR is generally
used as a response to DTR, by the Modem, to indicate that it
is ready. The signal acts only as a flag and does not control
any internal logic.