Legacy-Free Keyboard/Embedded Controller with SPI and LPC Docking Interface
5.2
FIFO Interrupt Mode Operation
5.2.1
RCVR Interrupts
When the RCVR FIFO and receiver interrupts are enabled (FCR bit 0 = "1", IER bit 0 = "1"), RCVR
interrupts occur as follows:
1. The receive data available interrupt will be issued when the FIFO has reached its programmed trigger
level; it is cleared as soon as the FIFO drops below its programmed trigger level.
2. The IIR receive data available indication also occurs when the FIFO trigger level is reached. It is
cleared when the FIFO drops below the trigger level.
3. The receiver line status interrupt (IIR=06H) has higher priority than the received data available
(IIR=04H) interrupt.
4. The data ready bit (LSR bit 0) is set as soon as a character is transferred from the shift register to
the RCVR FIFO. It is reset when the FIFO is empty.
5.2.2
RCVR FIFO Timeout Interrupts
When RCVR FIFO and receiver interrupts are enabled, RCVR FIFO timeout interrupts occur as follows:
1. A FIFO timeout interrupt occurs if all the following conditions exist:
■
at least one character is in the FIFO
■
The most recent serial character received was longer than 4 continuous character times ago. (If 2
stop bits are programmed, the second one is included in this time delay).
■
The most recent CPU read of the FIFO was longer than 4 continuous character times ago.
This will cause a maximum “character-received-to-interrupt-issued” delay of 160 msec at 300 BAUD with
a 12 bit character.
2. Character times are calculated by using the RCLK input for a clock signal (this makes the delay
proportional to the baud rate).
3. When a timeout interrupt has occurred it is cleared and the timer reset when the CPU reads one
character from the RCVR FIFO.
4. When a timeout interrupt has not occurred the timeout timer is reset after a new character is received
or after the CPU reads the RCVR FIFO.
5.2.3
XMIT Interrupts
When the XMIT FIFO and transmitter interrupts are enabled (FCR bit 0 = "1", IER bit 1 = "1"), XMIT
interrupts occur as follows:
1. The transmitter holding register interrupt (02H) occurs when the XMIT FIFO is empty; it is cleared
as soon as the transmitter holding register is written to (1 of 16 characters may be written to the
XMIT FIFO while servicing this interrupt) or the IIR is read.
2. The transmitter FIFO empty indications will be delayed 1 character time minus the last stop bit time
whenever the following occurs: THRE=1 and there have not been at least two bytes at the same
time in the transmitter FIFO since the last THRE=1. The transmitter interrupt after changing FCR0
will be immediate, if it is enabled.
Character timeout and RCVR FIFO trigger level interrupts have the same priority as the current received
data available interrupt; XMIT FIFO empty has the same priority as the current transmitter holding
register empty interrupt.
Revision 1.1 (01-14-03)
SMSC LPC47N350
DATA3S4HEET
SMSC [ SMSC CORPORATION ]
