Philips Semiconductors Product data sheet
SC28L194Quad UART for 3.3 V and 5 V supply voltage
2006 Aug 15
23
Table 11. ISR - Interrupt Status Register
Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
I/O Port
change of
state
Receiver
Watch-dog
Time-out
Address
recognition event
Xon/off
event
Always 0 Change of
Break State
RxRDY
Receiver has entered
arbitration process
TxRDY
Transmitter has entered
arbitration process
This register provides the status of all potential interrupt sources for
a UART channel. When generating an interrupt arbitration value, the
contents of this register are masked by the interrupt mask register
(IMR). If a bit in the ISR is a ’1’ and the corresponding bit in the IMR
is also a ’1’, interrupt arbitration for this source will begin. If the
corresponding bit in the IMR is a zero, the state of the bit in the ISR
can have no affect on the IRQN output. Note that the IMR may or
may not mask the reading of the ISR as determined by MR1[6]. If
MR1[6] is cleared, the reset and power on default, the ISR is read
without modification. If MR1[6] is set, the a read of the ISR gives a
value of the ISR ANDed with the IMR.
ISR[7] - Input Change of State
This bit is set when a change of state occurs at the I/O1 or I/O0
input pins. It is reset when the CPU reads the Input Port Register,
IPR.
ISR[6] Watch-dog Time-out
This bit is set when the receiver’s watch-dog timer has counted
more than 64 bit times since the last RxFIFO event. RxFIFO events
are a read of the RxFIFO or GRxFIFO, or the push of a received
character into the FIFO. The interrupt will be cleared automatically
upon the push of the next character received or when the RxFIFO or
GRxFIFO is read. The receiver watch-dog timer is included to allow
detection of the very last characters of a received message that may
be waiting in the RxFIFO, but are too few in number to successfully
initiate an interrupt. Refer to the watch-dog timer description for
details of how the interrupt system works after a watch-dog time-out.
ISR[5] - Address Recognition Status Change
This bit is set when a change in receiver state has occurred due to
an Address character being received from an external source and
comparing to the reference address in ARCR. The bit and interrupt
is negated by a write to the CR with command x11011, Reset
Address Recognition Status.
ISR[4] - Xon/Xoff Status Change
This bit is set when an Xon/Xoff character being received from an
external source. The bit is negated by a read of the channel Xon
Interrupt Status Register, XISR.
ISR[3] - Reserved Always reads a 0
ISR[2] - Change in Channel Break Status
This bit, when set, indicates that the receiver has detected the
beginning or the end of a received break. It is reset when the CPU
issues a reset break change interrupt command via the CR.
ISR[1] - Receiver Ready
The general function of this bit is to indicate that the RxFIFO has
data available. The particular meaning of this bit is programmed by
MR2[3:2]. If programmed as receiver ready(MR2[3:2] = 00), it
indicates that at least one character has been received and is
waiting in the RxFIFO to be read by the host CPU. It is set when the
character is transferred from the receive shift register to the RxFIFO
and reset when the CPU reads the last character from the RxFIFO.
If MR2[3:2] is programmed as FIFO full, ISR[1] is set when a
character is transferred from the receive holding register to the
RxFIFO and the transfer causes the RxFIFO to become full, i.e. all
sixteen FIFO positions are occupied. It is reset when ever RxFIFO is
not full. If there is a character waiting in the receive shift register
because the FIFO is full, the bit is set again when the waiting
character is transferred into the FIFO.
The other two conditions of these bits, 3/4 and half full operate in a
similar manner. The ISR[1] bit is set when the RxFIFO fill level
meets or exceeds the value; it is reset when the fill level is less. See
the description of the MR2 register.
Note: This bit must be at a one (1) for the receiver to enter the
arbitration process. It is the fact that this bit is zero (0) when the
RxFIFO is empty that stops an empty FIFO from entering the
interrupt arbitration. Also note that the meaning if this bit is not quite
the same as the similar bit in the status register (SR).
ISR[0] - Transmitter Ready
The general function of this bit is to indicate that the TxFIFO has an
at least one empty space for data. The particular meaning of the bit
is controlled by MR0[5:4] indicates the TxFIFO may be loaded with
one or more characters. If MR0[5:4] = 00 (the default condition) this
bit will not set until the TxFIFO is empty - sixteen bytes available. If
the fill level of the TxFIFO is below the trigger level programmed by
the TxINT field of the Mode Register 0, this bit will be set. A one in
this position indicates that at least one character can be sent to the
TxFIFO. It is turned off as the TxFIFO is filled above the level
programmed by MR0[5:4. This bit turns on as the FIFO empties; the
RxFIFO bit turns on as the FIFO fills. This often a point of confusion
in programming interrupt functions for the receiver and transmitter
FIFOs.
Note: This bit must be at a one (1) for the transmitter to enter the
arbitration process. It is the fact that this bit is zero (0) when the
RxFIFO is full that stops a full FIFO from entering the interrupt
arbitration. Also note that the meaning if this bit is not quite the same
as the similar bit in the status register (SR).
