SC28C94A1A,518 NXP Semiconductors, SC28C94A1A,518 Datasheet - Page 8

• Manufacturer Part Number: SC28C94A1A,518
• Description: IC UART QUAD W/FIFO 52-PLCC
• Manufacturer: NXP Semiconductors

Specifications of SC28C94A1A,518

• Features: False-start Bit Detection
• Number Of Channels: 4, QUART
• Fifo's: 8 Byte
• Voltage - Supply: 5V
• With Auto Flow Control: Yes
• With False Start Bit Detection: Yes
• With Modem Control: Yes
• With Cmos: Yes
• Mounting Type: Surface Mount
• Package / Case: 52-PLCC
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: 568-1114-2; 935262534518; SC28C94A1A-T

This continues regardless of issuance of the stop counter command.
Philips Semiconductors
set each time the counter/timer transitions from 1 to 0. (High to low)
ISR[3] is reset by the stop counter command. NOTE: Reading of
the CTU and CTL registers in the timer mode is not meaningful.
When the C/T is used to generate a baud rate and the C/T is
selected through the CSR then the receivers and/or transmitter will
be operating in the 16x mode. Calculation for the number ‘n’ to
program the counter timer upper and lower registers is shown below.
n=2 x 16 x Baud rate desired/(C/T Clock Frequency
Often this division will result in a non-integer number; 26.3 for
example. One can only program integer numbers to a digital divider.
Therefore 26 would be chosen. This gives a baud rate error of
0.3/26.3 which is 1.14%; well within the ability of the asynchronous
mode of operation.
Counter Mode
In the counter mode the counter/timer counts the value of the CTLR
CTUR down to zero and then sets the ISR[3] bit and sets the
counter/timer output from 1 to 0. It then rolls over to 65,365 and
continues counting with no further observable effect.
Reading the C/T in the counter mode outputs the present state of
the C/T. If the C/T is not stopped, a read of the C/T may result in
changing data on the data bus.
Timeout Mode
The timeout mode uses the received data stream to control the
counter. The time-out mode forces the C/T into the timer mode.
Each time a received character is transferred from the shift register
to the RxFIFO, the counter is restarted. If a new character is not
received before the counter reaches zero count, the counter ready
bit is set, and an interrupt can be generated. This mode can be used
to indicate when data has been left in the Rx FIFO for more than the
programmed time limit. If the receiver has been programmed to
interrupt the CPU when the receive FIFO is full, and the message
ends before the FIFO is full, the CPU will not be interrupted for the
remaining characters in the RxFIFO.
By programming the C/T such that it would time out in just over one
character time, the above situation could be avoided. The
processor would be interrupted any time the data stream had
stopped for more than one character time. NOTE: This is very
similar to the watch dog time of MR0. The difference is in the
programmability of the delay time and that the watchdog timer is
restarted by either a receiver load to the RxFIFO or a system read
from it.
This mode is enabled by writing the appropriate command to the
command register. Writing an ‘Ax’ to CRA or CRB will invoke the
timeout mode for that channel. Writing a ‘Cx’ to CRA or CRB will
disable the timeout mode. Only one receiver should use this mode
at a time. However, if both are on, the timeout occurs after both
receivers have been inactive for the timeout period. The start of the
C/T will be on the logical or of the two receivers.
The timeout mode disables the regular START/STOP counter
commands and puts the C/T into counter mode under the control of
the received data stream. Each time a received character is
transferred from the shift register to the RxFIFO, the C/T is stopped
after one C/T clock, reloaded with the value in CTUR and CTLR and
then restarted on the next C/T clock. If the C/T is allowed to end the
count before a new character has been received, the counter ready
bit, ISR[3], will be set. If IMR[3] is set, this will generate an interrupt.
Since receiving a character restarts the C/T, the receipt of a
character after the C/T has timed out will clear the counter ready bit,
ISR[3], and the interrupt. Invoking the ‘Set Timeout Mode On’
command, CRx=‘Ax’, will also clear the counter ready bit and stop
the counter until the next character is received.
2006 Aug 09
Quad universal asynchronous receiver/transmitter (QUART)
8
The counter timer is controlled with six commands: Start/Stop C/T,
Read/Write Counter/Timer lower register and Read/Write
Counter/Timer upper register. These commands have slight
differences depending on the mode of operation. Please see the
detail of the commands under the CTLR CTUR Register
descriptions.
Time–out Mode Caution
When operating in the special time–out mode, it is possible to
generate what appears to be a “false interrupt”, i.e., an interrupt
without a cause. This may result when a time-out interrupt occurs
and then, BEFORE the interrupt is serviced, another character is
received, i.e., the data stream has started again. (The interrupt
latency is longer than the pause in the data strea.) In this case,
when a new character has been receiver, the counter/timer will be
restarted by the receiver, thereby withdrawing its interrupt. If, at this
time, the interrupt service begins for the previously seen interrupt, a
read of the ISR will show the “Counter Ready” bit not set. If nothing
else is interrupting, this read of the ISR will return a x’00 character.
This action may present the appearance of a spurious interrupt.
Receiver and Transmitter
The QUART has four full-duplex asynchronous
receiver/transmitters. The operating frequency for the receiver and
transmitter can be selected independently from the baud rate
generator, the counter/timer, or from an external input.
Registers associated with the communications channel are the
mode registers (MR0, MR1 and MR2) Clock Select Register (CSR),
Command Register (CR), Status Register (SR), Transmit FIFO
(TxFIFO), and the Receive FIFO (RxFIFO). The transmit and
receive FIFOs are each eight characters deep. The receive FIFO
also stores three status bits with each character.
Transmitter
The transmitter accepts parallel data from the CPU and converts it
to a serial bit stream on the TxD output pin. It automatically sends a
start bit followed by the programmed number of data bits, an
optional parity bit, and the programmed number of stop bits. The
least significant bit is sent first. Following the transmission of the
stop bits, if a new character is not available in the TxFIFO, the TxD
output remains high and the TxEMT bit in the SR will be set to 1.
Transmission resumes and the TxEMT bit is cleared when the CPU
loads a new character in the TxFIFO. In the 16X clock mode, this
also re-synchronizes the internal 1X transmitter clock so that
transmission of the new character begins with minimum delay.
If the transmitter is disabled it continues operating until the character
currently being transmitted and any characters in the TxFIFO,
including parity and stop bits, have been transmitted. New data
cannot be loaded to the TxFIFO when the transmitter is disabled.
The transmitter can be forced to send a break (a continuous low
condition) by issuing a START BREAK command via the CR
register. The break is terminated by a STOP BREAK command or a
transmitter reset..
TxFIFO
The TxFIFO empty positions are encoded as a three bit number for
presentation to the bidding logic. The coding will equal the number
of bytes that remain to be filled. That is, a binary number of 101 will
mean five bytes may be loaded; 111 means 7, etc. Eight positions
will be indicated by a binary 111 and the FIFO empty bit will be set.
Receiver
The receiver accepts serial data on the RxD pin, converts the serial
input to parallel format, checks for start bit, stop bit, parity bit (if any),
SC28C94
Product data sheet