Integrated low voltage detect and watchdog circuit

A low voltage monitor and microcontroller output monitoring circuit for insuring proper operation of a microcontroller in a furnace control system. A switch connected to a reset port of the microcontroller is capable of putting either a high or low voltage signal at the reset port. A high signal is required for normal operation of the microcontroller. The low voltage monitor connects to a power supply providing power to the microcontroller and causes the switch to provide a low signal to the reset port if the output voltage of the power supply drops below a predetermined level. The output monitoring circuit insures that the microcontroller is operating properly by insuring that the microcontroller is producing an output signal at a predetermined frequency and repetition rate. If the output signal is not at the predetermined frequency, the output monitoring circuit causes a low voltage to be present at the reset port.

BACKGROUND OF THE INVENTION 
The present invention relates to the field of microcontroller and more 
specifically microcontrollers used in furnace control systems. 
Microcontrollers were widely used in heating and cooling control systems. 
In a furnace system, a microcontroller was responsible for operating, for 
example, valves, fans and igniters. The microcontroller also received 
inputs such as signals from limit switches. Due to the explosive nature of 
the gas used in furnaces, proper operation of the microcontroller was 
vital to insure safe operation of the furnace. Usually, in order to 
perform these functions, the microcontroller system included Read Only 
Memory (ROM) and Random Access Memory (RAM). 
The microcontroller typically operated according to the cycle indicated in 
FIG. 1. Once started at box A, the microcontroller went through an 
initialization process at box B. Interrupts were enabled at box C. Then, 
ROM was tested at box D. 
After the ROM test, the microcontroller would go into an operational loop. 
At box E, inputs were received. At box F, control functions were 
performed. At box G, outputs from the microcontroller were checked. At box 
H, RAM was tested. Lastly, at box I, the timers were checked to insure a 
constant time base signal was being generated. Then, the cycle would start 
over again at box E. The microcontroller also included a reset function 
(not shown) which causes the microcontroller to re-initialize when a 
signal at the reset port drops below a threshold voltage. 
It was important that the microcontroller operate correctly and produce the 
proper outputs to insure safe operation of the furnace. Microcontrollers 
required some source of power for operation. One problem which commonly 
caused improper operation of the microcontroller was a drop in the 
microcontroller's power supply. 
Another problem in the operation of the microcontrollers is that 
occasionally, the microcontroller may stop inadvertently at one step of 
its cycle and fail to proceed any further. This in turn affects the 
frequency and the repetition rate of the output signal 
Thus it is an objective of the present invention to provide a 
microcontroller monitor which resets the microcontroller when the power 
supply drops below a predetermined minimum voltage. It is a further 
objective of the present invention to provide a microcontroller monitor 
which resets the microcontroller when the microcontroller gets stopped 
inadvertently as it proceeds through its operational loop. 
SUMMARY OF THE INVENTION 
The present invention is a circuit for monitoring the power supply and the 
output from a microcontroller in a furnace system. The circuit includes a 
voltage monitoring means for monitoring the voltage supplied to the 
microcontroller, a microcontroller monitoring means for monitoring the 
output of the microcontroller, and a switch adapted to cause the 
microcontroller to reset if either the voltage monitoring means or the 
microcontroller monitoring means indicates that a problem exists.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
With reference to FIG. 2, thereshown is a furnace control system 5 which 
includes the present invention. Microcontroller 10 controls operation of a 
furnace (not shown) by following a preprogrammed set of instructions. The 
microcontroller includes two ports, an output port and a reset port. The 
output port puts out a signal which indicates that the microcontroller is 
operating properly. In the present case, this signal is a 1000 Hz square 
wave. The reset port is adapted to cause the microcontroller to restart if 
the voltage at the reset port drops below a predetermined level. Thus for 
normal operation of the microcontroller it is desirable to maintain a 
voltage at the reset port which is higher than the predetermined level. DC 
power is supplied to the microcontroller by an ac power supply, converted 
to dc in converter 95. 
Switch 15 is used to insure that the voltage at the reset port is at a high 
level during normal operation of the power supply and normal operation of 
the microcontroller. Here, switch 15 is comprised of a bipolar junction 
transistor (BJT) 20. Note that other types of transistors, and other 
voltage controlled switches such as relays could be used as substitutes 
for BJT 20. BJT 20 has a collector 21, a base 22 and an emitter 23. The 
collector is tied to a direct current power supply for biasing, through 
resistor 80. The emitter 23 is tied to the return side of the direct 
current power supply, which is ground in the present embodiment. The base 
22 is a control line for the switch. If the voltage at the base 22 is 
greater than a second predetermined voltage, BJT 20 turns on and the 
voltage at collector 21 is sunk to ground, thus causing the 
microcontroller 10 to reset. If the voltage at the base is kept below the 
second predetermined voltage, then BJT 20 is kept off and a high voltage 
remains on collector 21 and at the reset port. 
Voltage monitoring means 25 is connected between the control line of switch 
15 and the power source of the limit switches through resistor 85. The 
voltage monitoring means 25 is comprised of first diode 30 and first zener 
diode 35, each diode having a cathode and an anode. Here, the cathode of 
first diode 30 is connected to resistor 85, while its anode is connected 
to the anode of first zener diode 35. First zener diode 35 has a breakdown 
voltage in this embodiment of V.sub.BK =-24 volts, which is equal to the 
negative peak of the AC voltage supply. 
Microcontroller monitoring means 45 is connected between microcontroller 10 
and the control line of switch 15. One embodiment of the microcontroller 
monitoring means includes a resistor 50, first capacitor 60 having a 
cathode and an anode, second diode 65 having a cathode and an anode, third 
diode 60 having a cathode and an anode, second capacitor 75 having a 
cathode and an anode and second zener diode 40 a cathode and an anode. 
Resistor 50 has first and second ports, the first port being connected to 
the output port of the microcontroller 10. The anode of first capacitor 60 
is connected to the second port of resistor 50. 
The cathode of first capacitor 60 is connected to both the anode of first 
diode 65 and the cathode 70 of second diode 70. The cathode of first diode 
65 is connected to ground. The anode of second diode 70 is connected to 
the control line of switch 15 (the base 22 of BJT 20). 
The anode of capacitor 75 is also connected to the control line of switch 
15, while its cathode is connected to ground. 
The anode of second zener diode 40 is connected to the control line of 
switch 15, while its cathode is connected to ground. 
A preferred embodiment of the present invention uses the following parts: 
BJT 20 is a 2N3417 transistor, resistor 80 is 10,000 ohm resistor, 
resistor 85 is a 2000 ohm resistor, resistor 90 is a 82,000 ohm resistor, 
first zener diode 35 has a V.sub.BK =-24 volts, second zener diode has a 
V.sub.BK =-5.6 volts, resistor 50 is a 2000 ohm resistor and first and 
second capacitors 60, 75 are .1 farad capacitors. 
The operation of the circuit will now be described. As was stated earlier, 
to prevent microcontroller 10 from resetting, a high voltage must be 
maintained at the reset port. This in turn requires that a low voltage be 
maintained at the control line of switch 15, or in this case at the base 
22 of BJT 20. There are two criteria, either of which will insure that the 
voltage on the control line stays low. 
The first criteria is that the AC voltage source must go negative with 
respect to ground. This creates a path from ground through second 
capacitor 75, through first zener diode 35, through first diode 30, 
through resistor 85 back to the voltage source. If the voltage is large, 
first zener diode 35 will break down and current will flow. This will 
cause a negative voltage to appear across second capacitor 75, thus 
keeping BJT 20 in an off state and the microcontroller 10 operating 
normally. If the AC supply is too small, then first zener diode 35 will 
not break down and BJT 20 will be turned on, thus causing a low voltage at 
the reset line and resetting the microcontroller 5. 
The second criteria is that during the positive half cycle of the AC power 
supply, the microcontroller 5 must have a 1000 Hz output signal at the 
output port to prevent the microcontroller 5 from resetting. Software 
internal to the microcontroller allows the 1000 Hz signal to occur only 
during the positive half cycle of the AC power supply. Of course, the 
exact frequency is a matter of design choice and can be changed by 
changing the resistance of resistor 50 and/or the capacitance of first 
capacitor 60. When the output port is high, first capacitor 60 charges 
through resistor 50 and second diode 65. When the output port goes low, 
there is a path to shuttle the charge from first capacitor 60 to second 
capacitor 75. 
In order to take advantage of both features of the circuit, the 
microcontroller 5 must, in this embodiment, produce a 1000 Hz output 
signal during the positive half cycle of the AC power supply. This keeps 
second capacitor 75 negative, thus keeping the BJT 20 off and the voltage 
at the reset port high. On the negative half cycle of the AC power supply, 
the output port is set high and the AC power supply signal is responsible 
for keeping second capacitor 75 negative and the BJT 20 off. If the AC 
power supply signal drops, the second capacitor 75 will be charged 
positive by resistor 90 and cause the BJT 20 to be on, which in turn 
causes the microcontroller 5 to reset. If the microcontroller 5 quits 
sending out the 1000 Hz signal, second capacitor 75 will again charge 
positively and cause microcontroller 5 to reset. 
It is important to note that resistor 90 and second capacitor 75 are 
selected to have a time constant which insures that a reset can occur in 
less than one-half of a 60 Hz cycle if either the 1000 Hz signal or the AC 
power supply signal are missing. Also, second zener diode 40 is used to 
insure that second capacitor 75 can never be charged too negatively to 
operate during the microcontroller monitoring part of the cycle. 
The foregoing has been a description of the construction and operation of a 
novel integrated low voltage detect and watchdog circuit having few parts. 
The inventors do not intend to limit the invention to the foregoing 
description, but instead define the limits of the foregoing invention in 
the following claims.