Abstract:
A pulse generator for indicating a change in the magnitude of an input signal includes a comparator, a first network connected to one input of the comparator for receiving the input signal, and a second network connected to the other input of the comparator for receiving the input signal. Both the first and second networks provide output signals to the comparator which transition in response to a magnitude change of the input signal from their respective baseline magnitudes to respective peak magnitudes, and back to their respective baseline magnitudes. The component values of the networks are selected such that one of the first or second network output signals is positive relative to the other network output signal over some period of time during the transition to the peak magnitude and return to the baseline magnitude to cause the comparator to generate a pulse indicating a change in the magnitude of the input signal.

Description:
TECHNICAL FIELD 
     The present invention relates generally to a pulse generator for indicating a change in the magnitude of an input signal, and more particularly to a comparator circuit that generates an output pulse upon either positive or negative transitions of an input signal routed through first and second parallel networks to the positive and negative input terminals of the comparator. 
     BACKGROUND OF THE INVENTION 
     The pulse generator of the present invention may be employed to address changes in the automotive industry relating to cruise control systems. Specifically, most conventional cruise control modules are operated by the driver using at least three switches (resume, set speed, and accelerate). Each switch input is typically provided to the cruise module as an individual battery voltage input signal. In other words, if the cruise module detects a battery voltage on an input line corresponding to the resume switch, then the module carries out commands associated with the driver&#39;s intent to resume cruise. 
     There is, however, an increasing demand for analog switch inputs to cruise modules wherein all of the switch signal inputs are provided over a single input line. Each switch, when activated by the driver, results in a different voltage level on the input line to the cruise module. Accordingly, in order to carry out the driver&#39;s intent, the cruise module must interpret this voltage level as corresponding to a particular switch. 
     An A/D converter could be employed to decode the input switch voltage level, but the cost of such converters and associated electronics, as well as the relatively limited opportunities for expanded functionality, make such an approach undesirable. Alternatively, a plurality of staged comparator circuits could be used to decode the input voltage level. Comparator thresholds could be set such that only one comparator will produce an output signal for each of the possible switch input voltage levels. This approach is undesirable because of the relatively large number of components required for its implementation. 
     A more desirable modification to the conventional cruise module is to employ a switch processing microcontroller (with an on-board A/D converter) to decode the analog switch inputs. While switch processing microcontrollers are relatively expensive, the increased functionality of a microcontroller in addition to the primary microcontroller of the cruise module, and expanded flexibility for future designs outweigh the cost disadvantages. 
     In addition to increased cost, however, a second microcontroller results in increased radiated emissions due to clocking pulses associated with the microcontroller. Thus, the second microcontroller should include a “sleep” mode function, wherein the microcontroller essentially shuts down until it is needed to decode a switch input, thereby reducing radiated emissions. Accordingly, a circuit is needed for generating a “wake” signal (to activate the second microcontroller) each time the switch input voltage to the cruise module changes. 
     SUMMARY OF THE INVENTION 
     The present invention provides a pulse generator for indicating a change in the magnitude of an input signal, such as the analog switch input to a cruise control module, by outputting a pulse which may be used, for example, as a wake signal to activate a switch processing microcontroller as described above. The pulse generator includes a comparator, a first network connected to one input to the comparator, and a second network connected to the other input to the comparator. Each of the first and second networks are capacitively coupled to the analog switch input signal, and include a voltage divider network to scale the input signal. The second network further includes a low pass filter to slow the response of the second network to changes in magnitude of the input signal. Accordingly, the signal outputted by the first network to the first input of the comparator deviates from a baseline magnitude upon the occurrence of an input signal magnitude change more quickly and to a greater extent than does the signal outputted by the second network to the other comparator input. 
     Thus, when one of the cruise control switches is actuated, the analog switch input signal provided to the first and second networks drops, for example, from a first voltage to a second voltage. The response characteristics of the first and second networks are designated such that the first input to the comparator drops more quickly, and below the signal present at the second input to the comparator. If the first input is the positive input to the comparator, the pulse generator will output a pulse (the wake pulse) with a duration equal to the time during which the first input is less than the second input. Since the analog switch input signal is present at the cruise module input for a period of time which is greater than the duration of the wake pulse, the second microcontroller, once activated, can decode the analog switch input signal. After the input signal is decoded, the second microcontroller re-enters the sleep mode until another switch transition occurs. As such, the second microcontroller remains inactive except when it is needed to decode the analog switch input signal, thereby reducing the radiated emissions of the second microcontroller. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features will become more apparent and the present invention will be better understood upon consideration of the following description and the accompanying drawings wherein: 
     FIG. 1 is a block diagram of an application of the pulse generator according to the present invention. 
     FIG. 2 is a schematic diagram of a pulse generator according to the present invention. 
     FIGS. 3-5 are waveform diagrams illustrating the relationship between of signals present at various locations of the schematic of FIG.  2 . 
     FIG. 6 is a flow chart of an analog switch input signal decoding process. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The embodiments of the invention described herein are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Rather, the embodiments selected for description have been chosen to enable one skilled in the art to practice the invention. 
     FIG. 1 shows the basic components of a cruise control system incorporating a pulse generator according to the present invention. The cruise control system  10  generally includes an on-off switch  12 , a resume switch  14 , a set/accelerate switch  16 , and a cruise module  18 . Cruise module  18  is typically connected to various indicators and sensors (none shown). Cruise module  18  includes, among other things, a resistor  20 , a second .microcontroller  22  (for analog switch input signal decoding), and a pulse generator  24 . 
     As shown, one node  26  of on-off switch  12  is connected to a power source, such as the vehicle battery. The other node  28  is connected to input  30  of cruise module  18 . Input  30  is connected to pulse generator  24 , second microcontroller  22 , and resistor  20 . The other side of resistor  20  is connected to ground. Common node  32  of switch  12  is connected to node  34  of resume switch  14  and node  36  of set/accelerate switch  16 . Node  38  of resume switch  14  is coupled through resistor  40  to input  30  of cruise module  18 . Similarly, nodes  42  and  44  of switch  16  are coupled through resistors  46  and  48 , respectively, to input  30  of cruise module  18 . Pulse generator  24  is connected to second controller  22  by line  50 . 
     When the driver switches on/off switch  12  into the “on” position such that common node  32  contacts node  36 , power is applied to node  34  of resume switch  14  and node  36  of set/accelerate switch  16 . When the driver moves set/accelerate switch  16 , for example, such that node  36  contacts node  42 , power is applied to node  42  and the voltage divider network including resistor  46  and resistor  20  results in an analog switch input signal having a particular voltage level at input  30  to cruise module  18 . When the signal at input  30  transitions to the voltage associated with the above-described activation of set/accelerate switch  16 , pulse generator  24  outputs a wake pulse on line  50  to second controller  22  as described in greater detail below. Second controller  22  becomes active, reads the signal at input  30 , and provides a decoded version of that signal to other electronics (not shown) on cruise module  18 . The values of resistors  40 ,  46 ,  48  are selected such that each of the three possible settings of switches  14 ,  16  results in a voltage divider network with resistor  20  that provides a different voltage level to input  30  of cruise module  18 . 
     FIG. 2 shows the internal components of pulse generator  24  of FIG.  1 . Pulse generator  24  generally includes a comparator  60 , a first network  62 , and a second network  64 . First network  62  includes capacitor  66 , resistor  68 , and resistor  70 . Second network  64  includes capacitor  72 , resistor  74 , resistor  76 , resister  78 , and capacitor  80 . A pull-up resistor  82  is connected to line  50 . 
     Input  30  is routed to both first network  62  and second network  64 . The input signal is capacitively coupled by capacitor  66  to the voltage divider network of resistors  68  and  70  of first network  62 . The common node  84  of first network  62  is connected to the positive input  86  of comparator  60 . The input signal is also capacitively coupled by capacitor  72  to the voltage divider network of resistors  74  and  76  of second network  64 . The common node  88  of resistors  74  and  76  is connected to the low pass filter including resistor  78  and capacitor  80 . The output node  90  of the low pass filter is connected to the negative input  92  of comparator  60 . The output  94  of comparator  60  is connected by line  50  to second microcontroller  22  as shown in FIG.  1 . 
     FIG. 3 shows the analog switch input signal at input  30  over a time period during which one of the three possible switches of FIG. 1 is activated. Signal conditioning circuitry (not shown) results in a baseline magnitude of the input signal of approximately 9.5 volts. For purposes of this description, assume that at a time corresponding to 20 milliseconds on the graph of FIG. 3, the driver actuated resume switch  14  (FIG.  1 ). Resistor  40  forms a resistor network with resistor  20  which results in an input signal pulse  96 . Other electronics (not shown) connected to cruise module  18  limit the duration of pulse  96  to approximately 10 milliseconds. Pulse  96  has a leading edge  98  which transitions from the baseline magnitude of approximately 9.5 volts to a second magnitude of approximately 7.25 volts. Pulse  96  also includes a trailing edge  100  which transitions from the second magnitude to the first magnitude at approximately 30 milliseconds. 
     FIG. 4 is a graph showing the signals present at comparator inputs  86  and  92  (i.e., the outputs of first circuit  62  and second circuit  64 , respectively) as a result of applying pulse  96  as an input signal to pulse generator  24 . Prior to the application of pulse  96 , the signal level of negative input  92  is at its baseline magnitude of slightly less than 6 volts, while the baseline magnitude of positive input  86  is slightly higher at approximately 6 volts. Accordingly, comparator  60  outputs a normally high signal on line  50  to second controller  22 . 
     At 20 milliseconds, the negative going leading edge  98  of input signal pulse  96  (FIG. 3) is applied to first and second networks  62 ,  64 . The component values of first network  62  are selected such that the voltage present at common node  84  and, therefore, at positive input  86  of comparator  60 , responds quickly to transitions in the input signal. As shown in FIG. 4, the signal at positive input  86  transitions rapidly at 20 milliseconds from its baseline magnitude to a peak deviation magnitude of just over 4 volts. The fast response characteristics of first network  62  also results in a rapid decay of the signal at positive input  86  from its peak deviation magnitude back to its baseline magnitude. 
     The component values of second network  64  are selected such that its response to input signal magnitude changes is slower than that of first network  62 . Accordingly, for a fraction of a millisecond beginning at 20 milliseconds, negative input  92  is greater than positive input  86 . Accordingly, comparator  60  generates a wake pulse  102  at output  94  as shown in FIG.  5 . In this application, second microcontroller  22  receives wake pulse  102  on line  50  and exits the sleep mode upon detection of the negative going leading edge. Next, second microcontroller  22  initiates a decode sequence according to the flow chart of FIG. 6 to decode the analog switch input at input  30  to cruise module  18 . 
     Referring now to FIG. 6, microcontroller  22  begins a watchdog timer at block  104  before reading the analog switch input signal (in this example, pulse  96  of FIG. 3) present at cruise module input  30  as indicated by block  106 . Microcontroller  22  next decodes the input signal by converting it to a digital representation. Next, the watchdog timer is incremented at block  110 , and the status of the switches  14 ,  16  is checked at decision block  112 . If switches  14 ,  16  are not open, second microcontroller  22  continues the loop of reading the input signal, decoding the signal, servicing the watchdog timer, and checking the switch status until either each of switches  14 ,  16  are open or the watchdog timer times out. The timeout period is set such that a timeout should not occur before the input signal (such as pulse  96 ) returns to its baseline magnitude. At decision block  112 , microcontroller  22  determines whether pulse  96  remains at its second magnitude. So long as pulse  96  remains at approximately 7.25 volts, microcontroller  22  will conclude that switch  14  is still closed. After the positive going trailing edge  100  of pulse  96  occurs, microcontroller  22  will conclude at decision block  112  that switches  14 ,  16  are open and re-enter sleep mode at block  114 . 
     As shown in FIGS. 4 and 5, the positive going trailing edge  100  of pulse  96  results in a positive spike at inputs  86 ,  92  of comparator  60  at approximately 30 milliseconds. As was the case with the negative going spike, the signal at positive input  86  deviates to a maximum peak magnitude faster than the signal at negative input  92 . Additionally, the difference between the peak magnitude and the baseline magnitude of the signal at positive input  86  is greater than the difference between the peak magnitude and the baseline magnitude of the signal at negative input  92 . However, since signal  86  decays faster than signal  92 , for approximately 10 milliseconds beginning just after 30 milliseconds, negative input  92  is greater than positive input  86 . Accordingly, comparator  60  outputs a wake pulse  103  during this time period as shown in FIG.  5 . 
     Microcontroller  22  re-enters sleep mode after a predetermined period of time passes without receiving a wake pulse  103 . 
     As should be apparent from the foregoing, the component values of pulse generator  24  of FIG. 2 should be selected such that the minimum expected change in magnitude of the input signal (either positive or negative) will result in a signal at negative input  92  of comparator  60  which is greater for a period of time than the input present at positive input  86 . 
     Although the present invention has been shown and described in detail, the same is to be taken by way of example only and not by way of limitation. Numerous changes can be made to the embodiments described above without departing from the scope of the invention.