Philips Semiconductors
SC16C554/554D
Quad UART with 16-byte FIFO and infrared (IrDA) encoder/decoder
Product data Rev. 05 — 10 May 2004 17 of 55
9397 750 13132
© Koninklijke Philips Electronics N.V. 2004. All rights reserved.
characters as soon as received data passes the programmed trigger level. To clear
this condition, the SC16C554/554D will transmit the programmed Xon1,2 characters
as soon as receive data drops below the programmed trigger level.
6.8 Special feature software flow control
A special feature is provided to detect an 8-bit character when EFR[5] is set. When
8-bit character is detected, it will be placed on the user-accessible data stack along
with normal incoming RX data. This condition is selected in conjunction with
EFR[0-3]. Note that software flow control should be turned off when using this special
mode by setting EFR[0-3] to a logic 0.
The SC16C554/554D compares each incoming receive character with Xoff2 data. If a
match exists, the received data will be transferred to the FIFO, and ISR[4] will be set
to indicate detection of a special character. Although the Internal Register Table
(Tabl e 8 ) shows each X-Register with eight bits of character information, the actual
number of bits is dependent on the programmed word length. Line Control Register
bits LCR[0-1] define the number of character bits, i.e., either 5 bits, 6 bits, 7 bits or
8 bits. The word length selected by LCR[0-1] also determine the number of bits that
will be used for the special character comparison. Bit 0 in the X-registers corresponds
with the LSB bit for the receive character.
6.9 Hardware/software and time-out interrupts
Three special interrupts have been added to monitor the hardware and software flow
control. The interrupts are enabled by IER[5-7]. Care must be taken when handling
these interrupts. Following a reset, if the transmitter interrupt is enabled, the
SC16C554/554D will issue an interrupt to indicate that the Transmit Holding Register
is empty. This interrupt must be serviced prior to continuing operations. The LSR
register provides the current singular highest priority interrupt only. It could be noted
that CTS and RTS interrupts have lowest interrupt priority. A condition can exist
where a higher priority interrupt may mask the lower priority CTS/RTS interrupt(s).
Only after servicing the higher pending interrupt will the lower priority CTS/TRS
interrupt(s) be reflected in the status register. Servicing the interrupt without
investigating further interrupt conditions can result in data errors.
When two interrupt conditions have the same priority, it is important to service these
interrupts correctly. Receive Data Ready and Receive Time Out have the same
interrupt priority (when enabled by IER[0]). The receiver issues an interrupt after the
number of characters have reached the programmed trigger level. In this case, the
SC16C554/554D FIFO may hold more characters than the programmed trigger level.
Following the removal of a data byte, the user should re-check LSR[0] for additional
characters. A Receive Time Out will not occur if the receive FIFO is empty. The
time-out counter is reset at the center of each stop bit received or each time the
receive holding register (RHR) is read. The actual time-out value is 4 character time.
In the 16 mode for the PLCC68 package, the system/board designer can optionally
provide software controlled 3-State interrupt operation. This is accomplished by
INTSEL and MCR[3]. When INTSEL interface pin is left open or made a logic 0,
MCR[3] controls the 3-State interrupt outputs, INTA-INTD. When INTSEL is a logic 1,
MCR[3] has no effect on the INTA-INTD outputs, and the package operates with
interrupt outputs enabled continuously.
