Download The Fridge Door Monitoring and Alarm Circuits

The Fridge Door Monitoring and Alarm Circuits
The system is designed to monitor the state of a fridge door, open or closed, and to set an alarm bell
ringing if the door is open.
Input Interface
The circuit to monitor the door is presented
first and is shown to the right. A
mechanically operated switch on the door
opens and closes with the door to make or
break a circuit. The voltage at point A is 5V
if the switch and the door are open, when no
current flows through the switch; the voltage
is 0V if the switch and the door are closed
and a current flows. The resistor in the circuit
is purely to limit the current flow when the
switch is closed.
9V
Data bus line used determines which
bit of register gets value of “A”.
Data Bus
Line
A
Gnd
Read
The voltage at A is passed to the input of the
tri-state driver the output of which drives one
of the Data bus lines when the tri-state is
enabled. To input the state of the fridge door
requires the computer to enable the driver
and read the value of the Data Bus into a
register, where it can be analysed.
Mechanical
Door Switch
Active low output
Address Decoder
AddressBus
Address decoder logic defines address of interface
This can be performed by the CPU executing an instruction such as:lw
$8, 0($6)
// where $6 holds the address assigned to the interface
which generates a read cycle at the address specified and puts the data value read into the register.
In this read cycle, the address from the instruction is sent on the Address bus and is analysed by
address decoding logic at the input interface, which checks for the address assigned to the interface. If
the decoding logic identifies the address, it activates its output by setting it low. When the read
control signal goes active, as part of the read cycle, the OR gate has both inputs low and sets its output
low enabling the tri-state driver. The driver drives the attached Data bus line with the value of A, 0 or
1. This value is held on the bus line and read into the CPU at the end of the read cycle, and then
transferred into the destination register. The read control signal is then de-activated, disabling the tristate driver which ceases to drive the Data bus line.
Which bit of the register the data value ends up in is determined by which Data bus line is attached to
the tri-state driver. Once the value has been read other instructions can be executed to test the
appropriate bit of the register to determine whether it is 0 or 1, i.e. whether the fridge door is open or
closed.
The input interface is accessed just as a memory during a read. The CPU does not need to treat a read
of the input interface any differently from a read of a memory: it reads a data value from an address.
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Output Interface
The Output interface is shown in the
figure. The bell circuit consists of the
bell unit which rings when a current
flows through it. The power transistor
operates as a switch, which controls
whether a current flows through the bell
or not. A special "power" transistor is
used which can handle the high current
required to ring the bell.
The power transistor is controlled by the
output of the computer system: a 1
enables the transistor ringing the bell, a
0 disables it turning the bell off. The
signal from the computer is held in the
transparent d-type flip-flop and the
computer turns the bell on and off by
changing the value in the flip-flop. The
value from the flip-flop cannot drive the
bell directly, as it cannot provide
sufficient current to ring the bell.
Data bus line used determines which bit
of register carries value for bell on/off.
Data Bus
Line
D
9V
Bell
Power
Transistor
Q
WE
0V
WRITE
Active low output
Address Decoder
Address Bus
Address decoder logic defines address of interface
An output value can be written into the flip-flop just as a value is written into a memory. The flip-flop
write control is enabled during a write cycle generated by an instruction such as:sw
$8, 0($6)
// where $6 holds the address assigned to the interface
As with the read interface the address is put on the address bus and is recognised by the address
decoding logic in the same way as for the input interface, setting its output low. Next the data value is
put on to the Data bus. When the write control signal goes active in the middle of the write cycle, both
inputs of the OR-gate are low and its output goes low activating the write control input to the flipflop. The flip-flop is now enabled for writing and writes the data value from the Data bus line
attached to its D input into the flip-flop. This value appears on the flip-flop output. After a delay the
CPU sets the write control line inactive, de-activating the write signal to the flip-flop and setting the
flip-flop into storage mode. The CPU then moves on to the next instruction, while the output interface
holds the data value keeping the bell in the required state, ringing or not ringing.
The Data bus line that is connected to the D input of the flip-flop of the interface determines which bit
in the data value is written to the flip-flop.
The circuit looks just the storage part of a memory. Again, the processor does not need to treat a write
to the output interface any differently from a write to a memory: it just writes a data value to an
address.
The use of a flip-flop is required in order to hold the bell ringing for a long period so that the bell can
be heard, i.e. the output value has to be held for long enough for it to be recognised by the receiver. A
tri-state driver cannot be used as the data value would only be present for a fraction of the length of
the write cycle, a few 10s of nano-seconds in a modern microprocessor.
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Software to control the interface
Flowchart for Fridge Door
ENTRY
Input Data - ‘0’ or ‘1’ from Fridge Door Switch
(read from switch address)
Check if ‘0’ - door Closed or ‘1’ - door Open
Door
Open
?
yes
no
Set Bell to ring!
Turn Bell off!
(write 1 to bell address)
A flowchart for the operation
of the interfaces is shown. The
input interface is read, the
data value read in is checked
to see if the door is open: if
the door is open, the bell is
turned on; if the door is closed
the bell is turned off. The
MIPS for this flowchart is
shown.
In this example both the input
and output interfaces are
attached to the least
significant bit of the Data bus.
(write 0 to bell address)
Exit
Flowchart for Fridge Door
ENTRY
Input Data - ‘0’ or ‘1’ from Fridge Door Switch
(read from switch address)
Check if ‘0’ - door Closed or ‘1’ - door Open
Door
Open
?
yes
no
Set Bell to ring!
Turn Bell off!
(write 1 to bell address)
(write 0 to bell address)
Exit
Of course, for this simple system, a computer is not really required, the door being open could be
wired to ring a bell. However, it demonstrates the key factors in I/O circuits: tri-state drivers between
the input interface and the Data bus; flip-flops between the Data bus and the output interface. I/O
systems differ in the number of connections to the data bus, and the logic on the other side of the tristate drivers (for input interfaces) or of the flip-flops (for output interfaces).
An advantage of a computer controlled interface is the ability to modify the operation of the system
by modifying the program. Thus, for the fridge door, a programmable delay could be introduced to
prevent the bell ringing as soon as you open the door, but allows a moderate time for access to the
fridge, avoiding irritation to the user.
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