Hardware Reference
In-Depth Information
i gure 5.6b, which is a kind of
optimized synchronous version of the one-shot circuit. Because the solution in i gure
5.7c is also synchronous (all Moore machines are), would you expect the circuit that
implements this state machine to be equal or at least similar to that of i gure 5.6b?
(See exercise 5.5.)
As a i nal comment, let us consider the circuit of
5.4.4 Temperature Controller
Figure 5.8a shows a circuit diagram for a temperature controller of an air conditioning
system. In the upper branch, the room temperature is sensed by some type of tem-
perature sensor and converted to digital format by the ADC (analog-to-digital con-
verter), producing the signal T room . In the lower branch, the user, by means of two
pushbuttons ( up , dn ), selects the reference (desired) temperature, producing the signal
T ref . Depending on the values of these two signals, the controller core decides whether
to heat the room ( h = '1'), to cool it ( c = '1'), or to stay in the idle state.
Because mechanical switches are subject to bounces before they i nally settle in
the proper position, the pushbuttons must be debounced. However, debouncers are
timed circuits, thus requiring a timed (category 2) machine to be implemented. Such
machines are seen in chapter 8, so for now let us just consider that the proper value
is produced for T ref (the design of this block is treated in section 8.11.4). For example,
T ref could be selected in the 60°F to 90°F range with an initial value (on power-up,
dei ned by the reset signal) of 73°F, if degrees Fahrenheit are used, or in the 15°C to
30°C range with a default value of 23°C, if degrees centigrade are employed instead.
An important addition to the system is depicted in
i gure 5.8b, which consists of
a display accessed by means of a multiplexer. The display shows the room temperature
while the selection pushbutton ( sel , with no need for debouncing, not shown in the
i gure) is at rest ( sel = '0') or the reference temperature while it is pressed ( sel = '1').
A state machine for the controller core, using the Moore approach, is depicted in
i gure 5.8c.
Δ
T represents the system hysteresis, which is generally a i xed circuit
Figure 5.8
Temperature controller. (a) Overall circuit diagram. (b) Display driver. (c) State machine for the
controller core block.
Search WWH ::




Custom Search