What are control and status signals? Explain them.
Various type of control and status signals are as follows :
(i). Address latch enable (ALE) :-
- This is an output signal, used to give information of AD0 -AD7 contents.
- It is a positive going pulse generated during the first clock cycle of a machine cycle.
- When pulse is high it indicates that the contents of AD0 - AD7 are address. When it is low it indicates that the contents are data.
- The ALE signal is used to separate AD0 -AD7 (i.e demultiplex) to A0 - A7 and D0 - D7. To do this separation an external latch is connected to AD0 - AD7 lines and this latch is controlled by ALE signals.
(ii) Input output/memory (IO/M) :
- This is an output status signals, used to give information of operation to be performed with memory or I/O device.
- If IO/M = 0, the microprocessor is performing a memory related operation.
- If IO/M = 1, the microprocessor is performing an I/O device related operation.
(iii) Read (RD) :
- This is an active low signal.
- It is an output control signal that is used to read data from the selected memory location or an I/O location via data bus.
(iv) Write (WR) :
- This an active low signal.
- It is an output control signal used to write data to selected memory location or an I/O location via data bus.
- A low on this pin indicated that a operation performed is write operation.
(v) Status signal (S1 and S2) :
- These are output status signals used to give information of operation performed by microprocessor.
- When S0 and S1 is combined with IO/M we get status of all the machine cycles (operations) performed by 8085 as shown in table below :
Table
Status
signals
|
Operation
|
Control
signal used
|
||
IO/M
|
S1
|
S2
|
||
0
|
0
|
0
|
-
|
-
|
0
|
0
|
1
|
Memory
write
|
WR
|
0
|
1
|
0
|
Memory
read
|
RD
|
0
|
1
|
1
|
Opcode
fetch
|
RD
|
1
|
0
|
0
|
-
|
-
|
1
|
0
|
1
|
I/O
write
|
WR
|
1
|
1
|
0
|
I/O read
|
RD
|
1
|
1
|
1
|
Interrupt
acknowledge
|
INTA
|
Z
|
0
|
0
|
Halt
|
|
Z
|
X
|
X
|
Hold
|
|
Z
|
X
|
X
|
Reset
|
|
Where :
Z – Tristate ( High impedance condition ) X- Unspecified condition
|
||||
(vi) Ready :
- This is an active high input control signal.
- It is used by microprocessor to detect whether a peripheral is ready for the data transfer or not. If not the processor waits till the signal goes high.
- The main function of this pin is to synchronization the microprocessor 8085 with slower peripherals i.e. the microprocessor waits till the peripherals is not ready to accept/send the data.
Posts
