
Memory Map and Registers
12-9
Communication Ports
OUTPUT
LEVEL
Output FIFO Level
. Contents of this 4-bit field:
0000
2
(0): indicates an empty output FIFO.
0001
2
(1) through 0111
2
(7): indicates the number of full positions in the
output FIFO.
1111
2
(15): indicates a full output FIFO.
An empty output buffer (OUTPUT LEVEL = 0000
2
) sends an unlatched,
positive level-triggered interrupt (OCEMPTY = 1) to the CPU. When the CPU
or DMA coprocessor writes to the empty output FIFO, OCEMPTY is cleared
to 0 and remains in that state until the buffer is again empty. An output FIFO
with one or more empty levels also sends an unlatched, positive level-trig-
gered interrupt (OCRDY = 1) to the CPU and the DMA coprocessor. This
condition causes a READY/NOT READY signal to be generated when the
CPU or DMA coprocessor attempts to write to the output FIFO. See Section
12.6, Coordinating Communication Ports With the CPU and DMA Coproces-
sor on page 12-17, for details.
INPUT
LEVEL
Input FIFO level
. Contents of this 4-bit field:
0000
2
(0): indicates an empty input FIFO.
0001
2
(1) through 0111
2
(7): indicates the number of full positions in the input
FIFO.
1111
2
(15): indicates a full input FIFO.
A full input FIFO (INPUT LEVEL = 1111
2
) sends an unlatched, positive level-
triggered interrupt (ICFULL = 1) to the CPU. When the CPU or DMA
coprocessor reads from the full input FIFO, ICFULL is cleared to 0 and re-
mains in that state until the FIFO is again full. An input FIFO with one or more
full levels also an unlatched, positive level-triggered interrupt (ICRDY = 1)
to the CPU and the DMA coprocessor. This condition causes a READY/NOT
READY signal to be generated when the CPU or DMA coprocessor attempts
to read from the input FIFO.
Reserved
Undefined.
12.3.2 Input-Port Register
This read-only register contains the contents of position 0, the oldest value of
the input FIFO. If this register is written to, its contents remain unchanged.
Reading from an empty input FIFO causes the CPU or DMA operation to stall
and to halt the peripheral bus.
12.3.3 Output-Port Register
This write-only register interfaces to position 7 (the newest value) of the output
FIFO. If this register is read, its contents remain unchanged, and the value
read is undefined.