XR16C2850
Transmit and Receive Holding Register
IER Vs Receive FIFO Interrupt Mode Operation
The serial transmitter section consists of an 8-bit
Transmit Hold Register (THR) and Transmit Shift
Register (TSR). The status of the THR is provided in
the Line Status Register (LSR). Writing to the THR
transfers the contents of the data bus (D7-D0) to the
THR, providing that the THR or TSR is empty. The
THRemptyflagintheLSRregisterwillbesettoalogic
1 when the transmitter is empty or when data is
transferred to the TSR. Note that a write operation can
be performed when the transmit holding register
empty flag is set (logic 0 = FIFO full, logic 1= at least
one FIFO location available).
When the receive FIFO (FCR BIT-0 = a logic 1) and
receive interrupts (IER BIT-0 = logic 1) are enabled,
the receive interrupts and register status will reflect
the following:
A) The receive data available interrupts are issued to
the external CPU when the FIFO has reached the
programmed trigger level. It will be cleared when the
FIFO drops below the programmed trigger level.
B) FIFO status will also be reflected in the user
accessible ISR register when the FIFO trigger level is
reached. Both the ISR register status bit and the
interrupt will be cleared when the FIFO drops below
the trigger level.
The serial receive section also contains an 8-bit
Receive Holding Register, RHR. Receive data is
removed from the 2850 and receive FIFO by reading
the RHR register. The receive section provides a
mechanism to prevent false starts. On the falling edge
of a start or false start bit, an internal receiver counter
starts counting clocks at 16x clock rate. After 7 1/2
clocks the start bit time should be shifted to the center
of the start bit. At this time, the start bit is sampled, and
if it is still a logic 0, it is validated. Evaluating the start
bit in this manner prevents the receiver from assem-
bling a false character. Receiver status codes will be
posted in the LSR.
C) The data ready bit (LSR BIT-0) is set as soon as a
character is transferred from the shift register to the
receive FIFO. It is reset when the FIFO is empty.
IER Vs Receive/Transmit FIFO Polled Mode Op-
eration
When FCR BIT-0 equals a logic 1, resetting IER bits
0-3 enables the 2850 in the FIFO polled mode of
operation. Since the receiver and transmitter have
separate bits in the LSR, either or both can be used in
the polled mode by selecting respective transmit or
receive control bit(s).
DEVICE IDENTIFICATION
The XR16C2850 provides a Device Identification and
Device Revision code to distinguish the part from
others.
A) LSR BIT-0 will be a logic 1 as long as there is one
byte in the receive FIFO.
To read the identification number from the part, its is
required to set the baud rate generator divisor latch to
1 and then set the content of the baud rate generator
DLL and DLM registers to 0. Reading the content of
the DLM will provide 12 hex for XR16C2850 part and
reading the content of the DLL will provide the revision
of the part.
B) LSR BIT 1-4 will indicate if an overrun error
occurred.
C) LSR BIT-5 will indicate when the transmit FIFO is
empty.
D) LSR BIT-6 will indicate when both the transmit
FIFO and transmit shift register are empty.
Interrupt Enable Register (IER)
The Interrupt Enable Register (IER) masks the inter-
rupts from receiver ready, transmitter empty, line
status and modem status registers. These interrupts
would normally be seen on the 2850 INT output pin.
E) LSR BIT-7 will indicate any FIFO data errors.
Rev. 1.00P
18
EXAR [ EXAR CORPORATION ]
