ST16C580
(FIFO’s enabled, FCR bit-0 = logic 1, FCR bit-3 = logic
0) and when there are no characters in the transmit
FIFO or transmit holding register, the -TXRDY pin will
be a logic 0. Once active the -TXRDY pin will go to a
logic 1 after the first character is loaded into the
transmit holding register.
Thesebitsareusedtosetthetriggerlevelforthereceive
FIFOinterrupt.
An interrupt is generated when the number of charac-
ters in the FIFO equals the programmed trigger level.
However the FIFO will continue to be loaded until it is
full.
Receive operation in mode “0”:
When the 580 is in mode “0” (FCR bit-0 = logic 0) or
in the FIFO mode (FCR bit-0 = logic 1, FCR bit-3 =
logic 0) and there is at least one character in the
receive FIFO, the -RXRDY pin will be a logic 0. Once
active the -RXRDY pin will go to a logic 1 when there
are no more characters in the receiver.
BIT-7
BIT-6
RX FIFO trigger level
0
0
1
1
0
1
0
1
1
4
8
14
Transmit operation in mode “1”:
When the 580 is in FIFO mode ( FCR bit-0 = logic 1,
FCR bit-3 = logic 1 ), the -TXRDY pin will be a logic 1
when the transmit FIFO is completely full. It will be a
logic 0 if one or more FIFO locations are empty.
Interrupt Status Register (ISR)
The 580 provides six levels of prioritized interrupts to
minimize external software interaction. The Interrupt
Status Register (ISR) provides the user with six inter-
ruptstatusbits. PerformingareadcycleontheISRwill
provide the user with the highest pending interrupt
level to be serviced. No other interrupts are acknowl-
edged until the pending interrupt is serviced. When-
ever the interrupt status register is read, the interrupt
status is cleared. However it should be noted that only
the current pending interrupt is cleared by the read. A
lower level interrupt may be seen after rereading the
interrupt status bits. The Interrupt Source Table 6
(below) shows the data values (bit 0-5) for the six
prioritized interrupt levels and the interrupt sources
associated with each of these interrupt levels:
Receive operation in mode “1”:
When the 580 is in FIFO mode (FCR bit-0 = logic 1,
FCR bit-3 = logic 1) and the trigger level has been
reached, or a Receive Time Out has occurred, the -
RXRDY pin will go to a logic 0. Once activated, it will
go to a logic 1 after there are no more characters in the
FIFO.
FCR BIT 4-5: (logic 0 or cleared is the default condi-
tion, TX trigger level = 1)
These bits are used to set the trigger level for the
transmit FIFO interrupt. The ST16C580 will issue a
transmit empty interrupt when the number of charac-
ters in FIFO drops below the selected trigger level.
BIT-5
BIT-4
TX FIFO trigger level
0
0
1
1
0
1
0
1
1
4
8
14
FCR BIT 6-7: (logic 0 or cleared is the default condi-
tion, RX trigger level =8)
Rev.1.20
20
EXAR [ EXAR CORPORATION ]
