11
FN6140.1
June 28, 2005
Operating Modes
Mode 2 (Strobed Bidirectional Bus I/O)
This functional configuration provides a means for
communicating with a peripheral device or structure on a
single 8-bit bus for both transmitting and receiving data
(bidirectional bus I/O). “Hand shaking” signals are provided to
maintain proper bus flow discipline similar to Mode 1. Interrupt
generation and enable/disable functions are also available.
Mode 2 Basic Functional Definitions:
Used in Group A only
One 8-bit, bidirectional bus Port (Port A) and a 5-bit
control Port (Port C)
Both inputs and outputs are latched
The 5-bit control port (Port C) is used for control and
status for the 8-bit, bidirectional bus port (Port A)
Bidirectional Bus I/O Control Signal Definition
(Figures 11, 12, 13, 14)
INTR
- (Interrupt Request). A high on this output can be
used to interrupt the CPU for both input or output operations.
Output Operations
OBF
- (Output Buffer Full). The OBF output will go “l(fā)ow” to
indicate that the CPU has written data out to port A.
ACK
- (Acknowledge). A “l(fā)ow” on this input enables the three-
state output buffer of port A to send out the data. Otherwise,
the output buffer will be in the high impedance state.
INTE 1
- (The INTE flip-flop associated with OBF).
Controlled by bit set/reset of PC4.
Input Operations
STB
- (Strobe Input). A “l(fā)ow” on this input loads data into the
input latch.
IBF
- (Input Buffer Full F/F). A “high” on this output indicates
that data has been loaded into the input latch.
INTE 2
- (The INTE flip-flop associated with IBF). Controlled
by bit set/reset of PC4.
FIGURE 11. MODE CONTROL WORD
FIGURE 12. MODE 2
1
D7 D6 D5 D4 D3 D2 D1 D0
CONTROL WORD
1/0 1/0
1
1/0
PC2-PC0
1 = INPUT
0 = OUTPUT
PORT B
1 = INPUT
0 = OUTPUT
GROUP B MODE
0 = MODE 0
1 = MODE 1
PC7
OBFA
PC6
INTE
1
PA7-PA0
ACKA
IBFA
PC4
WR
INTE
2
RD
PC3
PC5
PC2-PC0
8
STBA
3
I/O
INTRA
MS82C55A, MQ82C55A