ST16C550CQ-0A-EVB Exar Corporation, ST16C550CQ-0A-EVB Datasheet - Page 14

• Manufacturer Part Number: ST16C550CQ-0A-EVB
• Description: EVAL BOARD FOR ST16C550 48TQFP
• Manufacturer: Exar Corporation

Specifications of ST16C550CQ-0A-EVB

• Interface Type: UART
• Data Bus Width: 8 bit
• For Use With/related Products: ST16C550
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

ST16C550
Transmit and Receive Holding Register
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
THR empty flag in the LSR register will be set to a logic
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 = at least one byte in FIFO /
THR, logic 1= FIFO/THR empty).
The serial receive section also contains an 8-bit
Receive Holding Register, RHR. Receive data is
removed from the ST16C550 and receive FIFO by
reading the RHR register. The receive section pro-
vides 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 assembling a false character. Receiver
status codes will be posted in the LSR.
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 ST16C550 INT output
pin.
IER Vs Receive FIFO Interrupt Mode Operation
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.
Rev. 5.01
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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.
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 ST16C550 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).
A) LSR BIT-0 will be a logic 1 as long as there is one
byte in the receive FIFO.
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.
E) LSR BIT-7 will indicate any FIFO data errors.
IER BIT-0:
Logic 0 = Disable the receiver ready interrupt. (normal
default condition)
Logic 1 = Enable the receiver ready interrupt.
IER BIT-1:
Logic 0 = Disable the transmitter empty interrupt.
(normal default condition)
Logic 1 = Enable the transmitter empty interrupt.
IER BIT-2:
Logic 0 = Disable the receiver line status interrupt.
(normal default condition)
Logic 1 = Enable the receiver line status interrupt.