CA2336184A1

CA2336184A1 - Method for software driven generation of multiple simultaneous high speed pulse width modulated signals

Info

Publication number: CA2336184A1
Application number: CA002336184A
Authority: CA
Inventors: Ihor A. Lys; Frederick M. Morgan
Original assignee: Individual
Current assignee: Signify North America Corp
Priority date: 1998-06-26
Filing date: 1999-06-25
Publication date: 2000-01-06
Also published as: WO2000001067A3; EP1090459A2; JP2002519989A; AU5312999A; WO2000001067A2

Abstract

Systems and methods can provide, in one aspect, a method for modulating the pulse width of control signals generated on a plurality of separate channels. In one practice, the methods described herein are suitable for execution on a microprocessor or micro controller platform that includes a timer interrupt mechanism which will generate an interrupt in response to a timer counting down a selected time interval or time period. In one practice, the timer is set to count down a period of time that is representative of a portion, or sub period, of the PWM cycle. Upon expiration of that time period, the timer executes an interrupt that causes the micro controller to enter an interrupt service routine (ISR) that can further modulate the PWM cycle of one or more signals.

Description

METHOD FOR SOFTWARE DRIVEN GENERATION OF MULTIPLE
SIMULTANEOUS HIGH SPEED PULSE WIDTH MODULATED SIGNALS
Cross-Reference to Related Applications This application claims priority to and incorporates by reference the following United States patent applications: Multicolored LED Lighting Method and Apparatus United States patent application, filed August 27, 1997, naming George Mueller and Ihor Lys as inventors; Digitally Controlled Light Emitting Diode Systems and Methods, United States provisional patent application, filed December 17, 1997, naming George Mueller and Ihor Lys as inventors; Multi-Color Intelligent Lighting, United States provisional patent application, filed December 24, 1997, naming George Mueller and Ihor Lys as inventors; Digital Lighting Systems, United States provisional patent application, filed March 20, 1998, naming Ihor Lys as inventor; and System and Method for Controlled Illumination, United States provisional patent application, filed March 25, 1998, naming George Mueller and Ihor Lys as inventors, United States patent application Serial No. 09/215,624 filed on December 17, 1998, United States patent application Serial No. 09/213,537 filed on December 17, 1998, United States patent application Serial No. 09/213,607 filed on December 17, 1998, United States patent application Serial No. 09/213,189 filed on December 17, 1998, United States patent application Serial No. 09/213,548 filed on December 17, 1998, United States patent application Serial No. 09/213,581 filed on December 17, 1998, United States patent application Serial No. 09/213,659 filed on December 17, 1998, United States patent application Serial No. 09/213,540 filed on December 17, 1998, and PCT
application No. US98/26853 filed on December 17, 1998.
Field of the Invention This application relates to systems and methods for the generation of pulse width modulated signals for lighting or for other purposes.
Back oun of The Invention Pulse width modulation (PWM) is a common technique for controlling the z amount of power that is delivered to a device, such as a motor or a lamp. One of the main benefits of PWM is that it is a technique that allows a digital system to achieve relatively high resolution control over the operation of a particular device.
For example PWM is a technique commonly employed in motor control to allow a digital signal to control the speed of a motor. Specifically, PWM takes advantage of the fact that a motor is an inductive element that acts like an integrator. Accordingly, the application to that motor of a pulsed digital signal results in the motor integrating over the square_ wave of the pulsed digital signal. By selecting the proper duty cycle and frequency for the PWM signal with respect to the time response of the particular motor under control, the pulsed signal will appear to the motor as a DC signal at a voltage level that is intermediate between the high and low voltage levels of the pulsed digital power signal.
Accordingly, pulse width modulation is a technique that allows a digital output signal to effectively achieve control over a device as if the digital output signal could be set to an intermediate voltage level.
Traditional methods of providing pulse width modulated signals include employing data processing systems that have software timers which control the pulse modulation of a particular output. However, although these systems can work quite well, it is often not cost effective to dedicate a data processing platform to the task of running timer modules for a PWM application.
Other techniques include interrupt driven processes in which a microprocessor, such as one having a watchdog timer, receives periodic interrupts at a known rate. Each time through the interrupt loop the processor updates one or more output pins, thus creating a pulse width modulated signal on each output pin: In this case, the rate at _ which a signal can be modulated is the clock speed multiplied by the number of cycles in the interrupt routine. For interrupt routines that control multiple channels, the number of instructions in the interrupt routine and therefore the number of clock cycles, can be quite large. Thus, the time period for executing the update instructions can be significant, and the PWM signal may have poor resolution, lacking fine grain control over the system. Still other techniques exist which are effectively combinations of the first two processes, software loops that contain a variable number of instructions for these techniques. The processor uses the hardware timer to generate a periodic interrupt, and then, depending on whether the pulse is to be very short or not, either schedules another interrupt to finish the PWM cycle, or creates the pulse by itself in the first interrupt routine by executing a series of instructions consuming a desired amount of time between two PWM signal updates. The difficulty with this method is that for multiple PWM channels it is very difficult to arrange the timer based signal updates such that they do not overlap, and then to accurately change the update times for a new value of PWM signals.
Accordingly, a method is needed to provide multiple PWM channels without the problems of these traditional methods. Moreover, there is a need to provide a PWM
technique that provides fine resolution over multiple channels, and be executed on an inexpensive micro controller or microprocessor platform.
Summar5r Of The Invention The present disclosure provides methods and systems to generate multiple channels of pulse width modulated signals for any purpose using software techniques at speeds exceeding those commonly achievable with traditional software synthesis.
In particular, the systems and methods described herein provide, in one aspect, a method for modulating the pulse width of control signals generated on a plurality of separate channels. In one practice, the methods described herein are suitable for execution on a microprocessor or micro controller platform that includes a timer interrupt mechanism which will generate an interrupt in response to a timer counting down a selected time interval or time period. In one practice, the timer is set to count down a period of time that is representative of a portion, or sub period, of the PWM
cycle. For example, the timer can be selected to count down one fourth the time period of one PWM cycle. Upon expiration of that time period, the timer executes an interrupt that causes the micro controller to enter an interrupt service routine (ISR).
The ISR can review a set of pre-computed values each of which represents the value for a modulated channel for a specific sub period of the PWM cycle. The ISR can then write to the I/O pins that are associated with the different modulated channels to set the level of the respective PWM signal on each of the I/O pins. The ISR can then determine for that duty cycle which channel is to be further modulated. For example, for a PWM signal that has been subdivided into four sub periods, the ISR can determine that for the first of these sub period, the ISR is to further modulate the PWM
signal on the first channel. Similarly, when the ISR occurs during the second sub period, the ISR
can modulate the second channel. In any case, once the ISR has determined which channel to modulate the ISR can modify the signal level on the I/O pin associated with that channel, and execute a delay loop to consume the desired amount of time to maintain the channel in its toggled state. Once the delay loop has been completed the ISR can execute another I/O write operation to toggle the modulated signal to its previous state. The ISR can then advance the sub-bookkeeping value to point to the next sub period, and terminate processing. Upon termination of the ISR, the microprocessor or controller platform can go back to normal operation with the timer still being set to trigger upon the lapse of a time interval associated with a sub period of the PWM cycle.
In one practice, the systems and methods described herein provide control systems that allow for the modulation of a lamp that includes a red, green, and blue component. Each color component of the lamp can be associated with a particular channel. Each channel can be separately modulated to control the amount of red, green, or blue in the composite light generated by the lamp. Accordingly, the systems and methods described herein allow for the modulation of the hue of light generated by a colored lamp.
Brief description of the fi~~s_ The systems and methods of invention will be understood more clearly from a review of certain illustrated embodiments, which include the following:
Fig. 1 depicts one embodiment of a system for modulating the hue of light generated by a color lamp;

Fig. 2 depicts pictorially one cycle of a PWM signal generated across three separate channels, each channel being associated with one color component of the lamp depicted in Fig. 1;
Fig. 3 depicts a flow chart diagram of an intemzpt service routine suitable for use with the system depicted in Fig. 1; and Fig. 4 depicts two PWM channels modulated by an interrupt service routine as shown in Fig. 3.
Detailed Description of the Preferred Illustrater~ Fmhnri;ment The invention will now be set forth with reference to certain illustrated embodiments, including control systems that allow for the modulation of the hue of colored light generated by a lamp having a red, green and blue component.
However, it will be understood by those of ordinary skill in the art that these illustrated embodiments are merely representative of the systems and methods that are provided by the invention and that other embodiments can be realized without departing from the scope of the invention. For example, the systems and methods described herein can further be employed for providing PWM control systems suitable for controlling the operation of a motor, including controlling the speed at which the motor rotates. Further, it will be understood that the systems and methods described herein can be employed for controlling the power delivered to an electronic device, including a traditional incandescent lightbulb, for controlling the brightness at which the light radiates. Other embodiments can be realized, and the actual implementation of these other embodiments can vary depending upon the application at hand.
Fig. 1 depicts one system 10 that includes a microcontroller element 12, an amplifier 14, and a colored light 18 that includes three colored lamps depicted as a red lamp 20, green lamp 22 and blue lamp 24. As shown in Fig. 1, the microcontroller 12 couples to the amplifier 14 via three separate channels. Similarly, the amplifier couples to the lamp 18 via three separate channels. Each of the three channels can be associated with one PWM signal that modulates the operation of a respective one of the colored lamps 20, 22 and 24. Each of the elements depicted in Fig. 1 cooperate to provide a system that can modulate the relative intensities of each of the colored lamps 20, 22 and 24 to control the overall hue of the light generated by the lamp 18.
S To this end, the system 10 depicted in Fig. 1 can include a microcontroller that can be a conventional microcontroller such as any of the controllers available _ commercially including those available from the Microchip Company, including any of the PIC family of microcontrollers including the PICmicro 16CXX/17CXX family of microcontrollers. Alternatively, the system 10 can include a microprocessor that can access an external memory device for receiving instructions for operating as a system according to the invention. It will further be understood that those of ordinary skill in the art that other configurations can be employed for providing a data processing platform capable of implementing the systems and methods described herein.
However, it will be understood by those of ordinary skill in the art that the systems and methods 1 S described herein provide for low-cost control systems that can be operated on lightweight inexpensive data processing platforms such as one time programmable microcontrollers.
As is generally known, the microcontroller 12 can include I/O pins, each of which can be employed for carrying a control signal, such as the PWM signals described herein. Each I/O pin can provide a channel for one of the colored lamp components 20, 22 and 24.
To aid in controlling the light 18, the system 10 includes an optional amplifier _ element 14. The amplifier element 14 can be a power amplifier that amplifies a digital control signal, such as a TTL logic level signal usually having low power. The optional amplifier 14 can act to increase the power level of the signal generated by the microcontroller to provide a signal capable of powering the lamp components 20, 22 and 24. The development of such power amplifiers is well-known in the art, and any suitable power amplifier can be practiced with the systems described herein without departing from the scope hereof. Moreover, it will be understood that the amplifier element l4 is optional and that in some embodiments, no power amplifier will be necessary as the power required to activate either of the lamp components can be sufficiently generated by the microcontroller 12, or by another platform capable of generating the PWM signals. It will further be understood that although the amplifier 14 is depicted as a separate element, the amplifier 14 could be integrated into the lamp component 18. Other suitable modifications to the system 10 for integrating or modifying the system 10 to accommodate the power needs of the devices under control can be practiced herewith without departing from the scope of the invention.
Figs. 2 and 3 depict pictorially the operation of the system 10. Specifically, Fig.
2 depicts the PWM signals that can be generated on the I/O pins of the microcontroller 12. The duty cycles shown in Fig. 2 can be generated, in part, by the action of an interrupt service routine (ISR) that operates, for example, as shown in the flow chart diagram of Fig. 3. Specifically, Fig. 2 shows that a period P can be subdivided into four sub periods, depicted in Fig. 2 as subperiods 1, 2, 3 and 4. The exemplary period of Fig.
2 lasts for about 1/5,000 seconds in length, with each subperiod being substantially one fourth of that duration. The actual duration of the time period will vary according to the application. Moreover, it will be understood that the systems described herein can be employed to generate systems with variable time periods P.
In one practice, the time period for each subperiod can be entered into a watchdog timer within the microcontroller. The watchdog timer can trigger an interrupt each time the subperiod lapses so that an interrupt is generated that causes the microcontroller 12 to enter an ISR associated with the timer interrupt. One such ISR is depicted in Fig. 3. Specifically, Fig. 3 shows an ISR process 50 that begins in a step 52_ wherein in response to an interrupt occurring, the microcontroller exits out of its current control program and enters an ISR that is associated with that interrupt and that will be executed until completion.
The design and development of such ISRs is well known in the art and follows from general principals in the art of computer science and embedded systems.
In particular, ISR 50 depicted in Fig. 4 includes a number of steps including step 52 wherein the ISR writes to a port, typically understood as an I/O port on the microcontroller. Similarly, in a step 56, the ISR chooses a channel or channels wherein these channels are to be further modulated, as will be described in detail below. The additional modulation takes place by operation of steps 58 through 62 wherein a write operation takes place to toggle the value of the channel and wherein a delay loop maintains the channel in this toggled state for a predetermined or preselected period of time. Once that time period has lapsed, the process 50 can proceed from step 60 to _ step 62 wherein a further write operation can take place to toggle the channel back to its original value. The steps of the ISR depicted in Fig. 4 will now be described in more detail with reference to Fig. 3 which depicts the modulation of three PWM
channels by a system that includes an ISR process such as the ISR process 50 depicted in Fig. 4.
To generate a plurality of PWM signals, e.g. N PWM signals, the processor of the micro controller can schedule an interrupt of at least N equal sub periods. Each sub period can be associated with a global PWM update procedure, and a vernier update procedure. The sub periods can be denoted by Pi where the first sub period is one, the second is 2 and so on. This is shown in Fig. 2 by the PWM cycle period 40 that shows the PWM cycle as subdivided into four sub periods. Also depicted in Fig. 2 is that the sub period corresponds to the entry of an ISR. This is depicted in Fig. 2 by the demarcations 44 that correspond to the beginning of each of the sub periods.
In each sub period, which begins with an interrupt, the ISR executes a series of operations that modulates the PWM signals of the channels 32, 34 and 38, each of which corresponds to a respective one of the red, green and blue lamps depicted in Fig. 1. To this end, the ISR can update all PWM signals by selecting from a set of pre-computed values corresponding to the specific sub period. This can entail a single array read, followed by a single write to update the multiple I/O pins on which the PWM
signals are generated. For example, Fig. 2 show that when the ISR entry occurs at the beginning of sub period 2, each PWM channel can be modified as a function of the pre-computed values. For example, Fig. 2 shows that upon entry into the ISR at the beginning of sub period 2, the system will toggle the values of channels 32, 34 and 38, such that channels 32 and 34 toggle from high to low, and channel 38 toggles from low to high.
_g_ In a next step, the ISR can cause the microcontroller to further modulate the signal on a selected one or ones of the PWM channels. In Fig. 2, this further modulation is shown as the toggling of one of the PWM channels for about one fourth of a sub-period. For example, the system in sub period 2 can further modulate the value of the PWM channel 34, such that the value of channel 34 toggles from low to high for about one quarter of the duration of the sub period 2. This further toggling can be described as a "vernier modulation." To accomplish this operation, the system can execute a write to the respective I/O pin or pins, execute a series of instructions consuming the desired amount of time, and then execute another update, typically an I/O write.
This vernier modulation can reduce or increase the PWM "on" of "off' time, by changing the state of the signal for a desired portion of the sub period.
There are two possible cases, either the global update places the signal in the "off' state and the vernier routine turns it "on" for a portion of the sub period, or the global update is "on" and the vernier routine turns the signal "off' for a portion of the sub period.
In a subsequent step, the system can advance a sub period bookkeeping value to point to the next sub period, such as for example sub period 3. This positions the system for entering into the ISR upon the next interrupt and modulating the next PWM
channel in the series. Using these methods each PWM channel can change multiple times per PWM period. This is advantageous because software can use this property to further increase the apparent PWM frequency, while still maintaining a relatively low interrupt rate.
The methods described herein reduce the amount of processor time employed for controlling the modulation period of the PWM channels. For an example, consider two signals A and B with a resolution of 20 counts programmed to 7 and 14 counts respectively. These signals could be depicted as shown in Fig. 4.
In this example the pre-computed update value at Pi=1 is both signals high, or "on." Signal A then spends some time in the "on" state, while the interrupt service routine continues to execute. Signal A then goes low or "off' in the vernier step at the WO 00/01067 PCTNS99/14555.
first "v", and the interrupt service routine executes time delay code during the time before restoring the signal to the on state at the second "v." For the depicted signal A, the vernier occurs for about three counts, including the interrupt time. The actual time between the multiple update at the beginning of the sub period and the vernier update need not be known, so long as the time spent between the vernier updates is the desired time. While the vernier updates are occurring, signal B which was switched on remains on and un-affected. When the second interrupt occurs, both signals are switched off, and the vernier routine now adds four additional counts to the period of signal B. In this example only 7 out of the 20 counts, or 35% of the processor time plus the time required for 2 interrupts, has been consumed.
Since only one vernier period is required per signal generated, increasing the number of periods per PWM cycle can generate non-uniform PWM waveforms at frequencies higher than those possible on most microprocessors' dedicated hardware PWM outputs for a large number of possible output codes. The microprocessor still executes interrupts at fixed intervals.
To change the duty cycles of the signals produced, the process described herein can asynchronously update any or all of the global or vernier values, in any order, without having to synchronize with the ISR, and without stopping the ISR. The ISR
does not have to change any variables which the main code changes or vice-versa. Thus there is no need for interlocks of any kind.
This software routine can thus utilize a single timer to generate multiple PWM
_ signals, with each signal having the resolution of a single processor cycle.
On a Microchip PIC microprocessor, for example, this allows three PWM signals to be generated with a resolution of 1024 counts, each corresponding to only a single instruction. This allows a PWM period of 1024 instruction cycles, i.e 4882Hz at a 20 MHZ clock. Furthermore, for counts between 256 and 792 the PWM waveform is a non-uniform 9765Hz signal, with much lower noise power than the processor's own straight PWM generator. Other processors and controllers can yield other PWM
cycles and timing.

While the invention has been disclosed in connection with the preferred embodiments shown and described in detail, various modifications and improvements thereon will become readily apparent to those skilled in the art. Accordingly, the spirit and scope of the present invention is to be limited only by the following claims.

Claims

A method of generating a plurality of pulse width modulated signals, comprising updating said plurality of pulse width modulated signals from a set of pre-computed values corresponding to a plurality of sub periods of a PWM
cycle, modifying one of the signals by executing a write to the I/O pins, executing a series of instructions consuming the desired amount of time, executing a second write to the I/O pins, and advancing the sub period bookkeeping value to point to the next sub period in the PWM cycle.
2. A method according to claim 1, wherein updating said plurality of pulse width modulated signals includes conducting a single array read, followed by a single write to update a plurality of multiple I/O pins on which the signals are generated.
3. A method according to claim 1, including providing a plurality of sets of pre-computed values, each set corresponding to a particular pattern of modulation.
4. A method according to claim 3, including allow a user to select from said plurality of pre-computed values, to provide user control over the selected pattern of modulation.
5. A method according to claim 1, including coupling said pulse width modulated signals to a plurality of lamps, each corresponding to a preselected color, whereby hue of a light generated by said plurality of lamps is controlled as a function of said pulse width modulated signals.
6. A system for modulating the pulse of a digital signal, comprising a memory having stored therein a set of pre-computed values, each being representative of a signal level for said digital signal, and a programmable device having a program memory and a timer, said program memory storing a set of instructions capable of directing said programmable device to monitor said timer, and in response to a signal generated by said timer, to access said set of pre-computed values for selecting a signal level for said digital signal, and an interrupt service routine capable of directing said device to further modulate said digital signal.
A system according to claim 6, wherein said programmable device comprises a microcontroller.
8. A system according to claim 6, wherein said programmable device comprises a microprocessor.
9. A system according to claim 6, wherein said interrupt service routine comprises a routine activated in response to a timer interrupt generated by said programmable device.
10. A system according to claim 6, further comprising a plurality of sets of pre-computed values for said digital signal.
11. A system according to claim 6, further comprising an amplifier for adjusting a characteristic of said digital signal.
12. A system according to claim 6, further comprising a lamp coupled to said digital signal, for allowing said digital signal to control an operating characteristic of said lamp.