82C55A
tST
STB
tSIB
IBF
tSIT
INTR
tRIB
tRIT
RD
tPH
INPUT FROM
PERIPHERAL
tPS
FIGURE 7. MODE 1 (STROBED INPUT)
INTR (Interrupt Request)
A “high” on this output can be used to interrupt the CPU
when and input device is requesting service. INTR is set by
the condition: STB is a “one”, IBF is a “one” and INTE is a
“one”. It is reset by the falling edge of RD. This procedure
allows an input device to request service from the CPU by
simply strobing its data into the port.
INTE A
Controlled by bit set/reset of PC4.
INTE B
Controlled by bit set/reset of PC2.
Output Control Signal Definition
(Figure 8 and 9)
OBF
- Output Buffer Full F/F). The OBF output will go “low”
to indicate that the CPU has written data out to be specified
port. This does not mean valid data is sent out of the part at
this time since OBF can go true before data is available.
Data is guaranteed valid at the rising edge of OBF, (See
Note 1). The OBF F/F will be set by the rising edge of the
WR input and reset by ACK input being low.
ACK
- Acknowledge Input). A “low” on this input informs the
82C55A that the data from Port A or Port B is ready to be
accepted. In essence, a response from the peripheral device
indicating that it is ready to accept data, (See Note 1).
INTR
- (Interrupt Request). A “high” on this output can be
used to interrupt the CPU when an output device has
accepted data transmitted by the CPU. INTR is set when
ACK is a “one”, OBF is a “one” and INTE is a “one”. It is
reset by the falling edge of WR.
INTE A
Controlled by Bit Set/Reset of PC6.
INTE B
Controlled by Bit Set/Reset of PC2.
NOTE:
1. To strobe data into the peripheral device, the user must operate
the strobe line in a hand shaking mode. The user needs to send
OBF to the peripheral device, generates an ACK from the pe-
ripheral device and then latch data into the peripheral device on
the rising edge of OBF.
MODE 1 (PORT A)
CONTROL WORD
D7 D6 D5 D4 D3 D2 D1 D0
1
0
1
1
1/0
PC4, PC5
1 = INPUT
0 = OUTPUT
INTE
A
PA7-PA0
PC7
PC6
8
OBFA
ACKA
PC3
WR
PC4, PC5
2
INTRA
MODE 1 (PORT B)
CONTROL WORD
D7 D6 D5 D4 D3 D2 D1 D0
1
1
0
INTE
B
PC2
ACKB
PB7-PB0
PC1
8
OBFB
PC0
WR
INTRB
FIGURE 8. MODE 1 OUTPUT
9
HARRIS [ HARRIS CORPORATION ]
