When the XMIT FIFO and transmitter interrupts are enabled (FCR bit 0 = "1", IER bit 1 = "1"), XMIT interrupts occur as
follows:
A. 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.
B. 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.
FIFO POLLED MODE OPERATION
With FCR bit 0 = "1" resetting IER bits 0, 1, 2 or 3 or all to zero puts the UART in the FIFO Polled Mode of operation.
Since the RCVR and XMITTER are controlled separately, either one or both can be in the polled mode of operation. In
this mode, the user's program will check RCVR and XMITTER status via the LSR. LSR definitions for the FIFO Polled
Mode are as follows:
Bit 0=1 as long as there is one byte in the RCVR FIFO.
Bits 1 to 4 specify which error(s) have occurred. Character error status is handled the same way as when in the
interrupt mode, the IIR is not affected since EIR bit 2=0.
Bit 5 indicates when the XMIT FIFO is empty.
Bit 6 indicates that both the XMIT FIFO and shift register are empty.
Bit 7 indicates whether there are any errors in the RCVR FIFO.
There is no trigger level reached or timeout condition indicated in the FIFO Polled Mode, however, the RCVR and XMIT
FIFOs are still fully capable of holding characters.
Table 30 - Baud Rates
DESIRED
BAUD RATE
50
DIVISOR USED TO
GENERATE 16X CLOCK
PERCENT ERROR DIFFERENCE
HIGH
BETWEEN DESIRED AND ACTUAL1
SPEED BIT2
2304
1536
1047
857
768
384
192
96
64
58
48
32
24
16
12
6
3
2
1
0.001
-
-
0.004
-
-
-
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
1
75
110
134.5
150
300
600
1200
1800
2000
2400
3600
4800
7200
-
-
0.005
-
-
-
-
9600
-
-
19200
38400
57600
115200
230400
460800
0.030
0.16
0.16
0.16
0.16
32770
32769
1
Note1: The percentage error for all baud rates, except where indicated otherwise, is 0.2%.
Note 2: The High Speed bit is located in the Device Configuration Space.
Page 63
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