JP2005522089A - Optical communication transceiver and data transmission / reception method - Google Patents

Abstract

An optical communication transceiver includes an LED coupled in series with a resistor.
The microprocessor has one I / O pin connected to the LED. In the first mode, that is, the transmission mode, the LED emits light by periodically driving with forward bias and transmits data. In the second or receive mode, the LED is periodically driven with something other than reverse bias, eg, reverse bias or zero bias. Subsequently, in the LED, the charge amount of the electrostatic capacitance at the joint portion of the LED is changed using the photocurrent. The change in the amount of charge is measured using a timer. When the change in charge exceeds a predetermined threshold, the input light is read. Thus, the LED can be used to receive data in the second mode.

Description

  The present invention relates generally to light emitting diodes (LEDs), and more particularly to LEDs used for two-way optical communication.

  Light emitting diodes (LEDs) are inexpensive and are widely used as light sources. Its various applications include numeric displays, flashlights, liquid crystal backlights, vehicle brake lights, traffic signals, backlights, and power-on indicator lights throughout almost every electronic device and current appliance.

  Since LEDs are most often used as light emitters, it is easy to forget that they can also operate as photodiodes, or photodetectors. Most LEDs are designed as light emitters and not as photodetectors, but all LEDs can operate in virtually any mode.

  This compatibility between solid state emission and light detection was first described in the 1970s, but has since been largely forgotten by LED users. Mims "Siliconnections: Coming of Age in the Electronic Era" McGraw-Hill, New York, NY, 1986, and Mims "LED Circuits and Projects" Howard W. Sams and Co., Inc., New York, NY, 1973 Please refer to.

  Light emitting diodes emit light in a fairly narrow frequency band when a small current is applied to the diode in the proper direction, ie forward bias. Because its current-voltage characteristics are exponential, it is difficult to control the voltage applied directly across the LEDs with sufficient accuracy to obtain the desired current.

  Therefore, some means must be provided to limit the current. In discrete electronic circuit systems, this is usually done by placing a resistor in series with the LED. Most microprocessor I / O pins can sink more current than can be supplied, and the configuration shown in FIG. 1 is the most common way to drive LEDs from a microprocessor or microcontroller.

  FIG. 1 shows a typical prior art LED emitter circuit 100. The I / O pin 101 of the microprocessor controller 100 is used to sink current through the LED 102 using a resistor 103 that limits the amount of current.

  One important application that uses LEDs is optical signal communication. In most prior art optical communication applications, LEDs are used in the transmitter and photodiodes are used in the receiver. In addition, each component is typically driven individually by a dedicated circuit. Photodiodes are most often designed specifically to receive light signals in a specific narrow frequency range. Most photodiodes cannot emit light. As a result, there will be one circuit that drives the transmitter and another circuit that drives the receiver. This increases the cost and complexity of the communication system.

Accordingly, it is desirable to provide a light emitting diode that can be used for both a transmitter and a receiver in an optical communication system.
The optical communication transceiver includes an LED coupled in series with a resistor. The microprocessor has at least one I / O pin connected to the LED. In the first mode, that is, the transmission mode, the LED emits light by periodically driving with forward bias and transmits data. In the second or receive mode, the LED is periodically driven at a non-reverse bias, eg, reverse or zero bias, followed by photocurrent to charge the capacitance of the LED's junction (capacitance). Is changed. The change in the amount of charge is measured using a timer. When the change in charge exceeds a predetermined threshold, the input light is read. Thus, the LED can be used to receive data in the second mode.

Dual Pin LED Data Transceiver FIG. 2 shows an LED emitter / detector circuit according to the present invention. Here, an LED 202 and a resistor 203 are coupled in series between two I / O pins 201 of a microprocessor or microcontroller 200. In this case, both ends of the LED / resistor circuits 202-203 are connected to the microprocessor 200. Using conventional programming techniques, the I / O pins can be set low (0V), high (5V), or the pins can be used as inputs.

action mode
FIGS. 3a-3c each illustrate how this circuit can operate in three modes: forward bias or “light emission”, non-forward bias or “reverse bias”, and “discharge” or read. . In the light emission mode of FIG. 3a, the LED operates as usual and emits light. The emitted light can be modulated to transmit data. In the reverse bias mode of FIG. 3b, the normal emission polarity is switched to reverse bias the diode junction. Subsequently, by releasing one end in the discharge mode of FIG. 3c, i.e., setting one end as an input to the microprocessor, the optically generated photocurrent is the amount of received or read light. Can be optically discharged from the joint at a rate proportional to. If the reading light is modulated, data can be received. Capacitive discharge can be easily measured. Since Q = CV and C is known, measuring the change in the amount of charge effectively measures the change in voltage.

  At some point, the voltage applied to the input pin exceeds a predetermined input threshold T. By measuring how long this takes, high-resolution measurement of the reading light level is performed. This time measurement can be simply made by a counter 210 or clock signal in the microprocessor 200. For example, a small program loop increments instead of counter 210 until threshold T is exceeded.

  The circuit according to the invention requires no additional components and consumes very little power during reading. By switching between the light emitting mode and the reading mode, the LED can operate as both a transmitter and a receiver (transmitter / receiver) in the optical communication network.

  FIG. 4 shows two such transceivers 401-402 connected by an optical link 403. The link 403 can be any transparent medium such as air or fiber optic cable.

Single Pin LED Transceiver Surprisingly, it is also possible to construct a single LED transceiver by using only a single I / O pin of the microprocessor, as shown in FIG.

  As shown in FIG. 5, the microprocessor or microcontroller 500 has a single I / O pin 501 connected to the input of the LED 502, and the output of the LED is connected to a current limiting resistor 503. In this circuit, it is impossible to reverse bias the LED 502 as in the case of the circuit of FIG. Instead, the LED is shorted to zero bias by setting the I / O pin 501 low.

  Subsequently, the pin 501 is set to input. The input charges the capacitance of the LED junction when the photocurrent induced by the incident light is read. This continues until the voltage across the LED biases the junction forward enough to effectively use up the total photocurrent in the LED. If this voltage is allowed to exceed a predetermined digital input threshold, a basic timing technique similar to that described above can be used to receive the data.

  However, this is a difficult limitation. Standard red, green, orange and yellow LEDs are typically “on” at about 1.5V to 2V, which is generally more than the digital input threshold in 5V systems such as the microprocessor 500. Low. However, blue LEDs, and some recent high-brightness LEDs, may have a forward voltage drop of about 3V, which is high enough to be charged above the input threshold. Lower voltage systems, such as 3V systems and lower, have a lower input threshold and are therefore more amenable to this technique.

Also, the circuit of FIG. 5 is generally superior to the circuit of FIG. This is because the threshold is often biased closer to ground than the supply voltage V _DD . It should be noted that this one-pin version can be operated by being connected to the I / O pin 501 in the reverse order of resistance as in the case of FIG.

  The advantage of this pin transceiver is that any suitable LED indicator driven by a single pin of the microprocessor simply changes the firmware or software to operate as described above, this time as the same number of transceivers. Is also able to work. No hardware modification is required. Thus, a software change makes it easy to upgrade a standard LED indicator to also function as a transceiver. This embodiment is also suitable for systems where the number of I / O pins is limited.

Bidirectional communication In one communication application, two non-synchronized transceivers are phase synchronized with each other to exchange pulse width modulated data in both directions. In this protocol, the two receivers operate alternately in transmit and receive mode, with a relatively short light pulse indicating 0 or a space state and a relatively long light pulse indicating 1 or a mark state. Show.

Idle Cycle This protocol begins with an idle cycle in which the transceiver performs an idle cycle. In the idle cycle, there is a 4 millisecond reception period after the transceiver transmits a 1 millisecond optical pulse. During the reception period, the transceiver performs multiple light measurements. These light measurements provide only 1-bit resolution, ie whether the incoming light flux is above or below a predetermined threshold (nominal about 1.5V).

The synchronized loop idling cycle continues until at least two consecutive measurements indicate “light seen”. At this point, the transceiver assumes that an optical pulse coming from the counterpart transceiver has been detected and transitions from the idlin group to a slightly faster synchronized loop. During the locked loop, the transmitted light pulse is still on for 1 millisecond, but is followed by an indefinite number of light measurements. During the synchronous loop, the microprocessor terminates its measurement set either after a predetermined number of measurements or when a falling edge of an optical pulse is detected. A falling edge is considered detected when there is no “light detection” in 10 measurements after both pairs of consecutive measurements indicate “light detection”.

  Thus, the execution pattern in the synchronous loop is that one transceiver LED is on for 1 millisecond, followed by both transceiver LEDs off for 1 millisecond, and then the other transceiver LED is 1 It consists of turning on for milliseconds and finally turning off both LEDs for 1 millisecond. Even if the transceivers have a clock frequency error of up to 25%, they can still be synchronized. The nominal synchronous loop pulse rate is 250 Hz and the duty cycle is 25%.

Data communication During communication, data bits are transmitted in an asynchronous format. For example, an optical pulse of 1 millisecond indicates MARK, and an optical pulse of 0.5 millisecond indicates SPACE. The system typically idles while sending the MARK bit. Here, the operation of the data transfer loop is the same as that of the synchronous loop. During data transmission, the format is at least 16 MARK bits to enable synchronization, followed by 1 SPACE bit as a start bit, followed by 8 bits of data, followed by 1 as a stop bit. There is a MARK. This is similar to the general 8-N-1 RS-232 format.

  In order to decode the optical pulse, the receiving transceiver maintains a count of the number of “light detection” measurements for each execution of the synchronization loop. SPACE is recorded when the number of photodetection measurements counted is 7 or less, and MARK is recorded when the number of counted pulses is 8 or more. Normal asynchronous deframing (ie, dropping the leading SPACE start bit and the trailing MARK stop bit) can be performed. The resulting 8-bit data word is then available for application level programming. Simple data communication can also be combined with error correction and encryption. Other optical communication protocols are possible.

  As shown in FIG. 6, two transceivers 601 exchange optical modulation data through a double convex lens 602 to provide an electrically isolated communication link. In this case, a data rate exceeding 1 MHz can be achieved.

Programmable Key The transceiver according to the invention can also be used as a programmable key and a programmable lock. Many other technologies are used in intelligent keys (eg, RFID, card keys, etc.), but the transceiver according to the present invention does not require physical contact and thus is not worn unlike some card key systems. There is no magnetic stripe. In addition, unlike the RF system, directivity can be provided and the reach distance can be shortened, so that the user can completely control what is unlocked. This allows a single key to be used for many different locks without the possibility of unlocking the wrong lock simply because it is in the vicinity. Because the transceiver is bidirectional in nature, challenge / response and encryption protocols can be used, which can make key duplication and forgery very difficult. The visibility of the LED allows a user interface. At a minimum, the user can easily determine whether the transceiver is operating, that is, whether the battery is dead. Furthermore, when using the key, the transceiver also acts as a flashlight at the same time.

  Perhaps the most interesting advantage is that the transceiver is capable of peer-to-peer communication. Any transceiver can communicate information or permissions to the other transceiver. In this case, the transceiver can know the unlocking code and communicate the code to other transceivers. This ability to convey information is unique and not the ability of smart cards or RFID tags.

Authentication and security In some applications, a peer-to-peer capability to transfer information or authorization is desirable. In other applications, such as secure transactions such as finance, authentication is as important as the data transfer itself and must be prevented from communicating permissions without control. An unfortunate side effect associated with transceiver programmability is that there is no guarantee that if a "do not forward" data tag is inserted by the application, the partner transceiver will respect it. Non-transferable permissions and non-forgeable identification credentials are difficult problems with many subtleties.

  However, simple encryption is possible and can be used to keep transceiver transactions safe from eavesdropping and spoofing. The microprocessor used has enough power to implement a normal symmetric encryption algorithm. Since such an algorithm requires the transmitter and receiver to share a secret key, communication between any two transceivers is pre-configured. The transceiver can be equipped with sufficient memory to hold a large number of symmetric encryption keys and can therefore be configured to communicate with a number of other transceivers.

Zero Knowledge Proof Zero Knowledge Proof (ZKP) and public key (ie, asymmetric) cryptography allow a transceiver to securely prove its identity and to communicate with a transceiver that has access to the published information. Yes (see Schneier “Applied Cryptography” 2nd edition, John Wiley and Sons, New York, NY, 1996, pp. 101-111). A shared secret is not required.

  With the transceiver according to the invention, any LED can be easily converted into a communications transceiver. This has broad implications. This is because LEDs are widely used as power indicators in microprocessor-type transceivers. Since the indicator is typically connected through a microprocessor rather than directly connected to a power source, a minimal user interface (eg, some blinking) is available.

  In the following, several application examples in which the LED transceiver according to the present invention can be used will be described.

  The CRT monitor can blink its power lamp to indicate a low power "sleep" state. Modern CRT monitors typically also have a USB, both of which control the monitor settings. By adding a transceiver circuit according to the present invention, a complete data path can be provided from the power LED to a nearby computer, allowing the transceiver to be used as a key as described above. It can be used instead of or in addition to a password for logging in to a computer, or it can be used as a cryptographic authentication transceiver for electronic commerce. Similar techniques can be used with keyboard indicator lights.

  The transceiver allows the user to copy the complete diagnostic status of the failed instrument through the power on LED and send the diagnostic information to the service site. No special display or connector is required for the instrument.

  The transceiver can be used for exchanging personal information such as a telephone number using a power indicator such as a mobile phone or PDA or an LED backlight. One interesting application is that the toys can “communicate” with each other if the transceiver is embedded in a toy (eg, a stuffed animal).

  While the invention has been described by way of examples of preferred embodiments, it is to be understood that various other adaptations and modifications can be made within the spirit and scope of the invention. Accordingly, the scope of the appended claims is to encompass all such variations and modifications as fall within the true spirit and scope of the present invention.

1 is a schematic diagram of a prior art light emitter circuit. FIG. FIG. 3 is a schematic diagram of an LED emitter / detector circuit according to the present invention. FIG. 3 illustrates the circuit of FIG. 2 operating with forward bias. FIG. 3 illustrates the circuit of FIG. 2 operating with non-forward bias. FIG. 3 shows the circuit of FIG. 2 operating in a discharge mode. It is a figure which shows the some LED type | mold transceiver which couple | bonds and forms a communication network. FIG. 4 is a diagram of another embodiment of an LED emitter / detector circuit using a single I / O pin according to the present invention. It is a figure which shows that two transmitter / receivers exchange light modulation data through a double convex lens.

Claims (14)

Means for periodically driving the LED with forward bias and transmitting data by emitting light;
A means for periodically driving the LED with a non-forward bias, wherein after the LED is driven with a non-forward bias, the charge amount of the capacitance of the LED is optically changed to measure the light level. And a means for receiving data.   The optical communication transceiver according to claim 1, wherein the LED is driven with a reverse bias, and then the light level is measured by capacitively discharging a photocurrent.   The optical communication transceiver according to claim 1, wherein the LED is driven with zero bias, and then the light level is measured by capacitively charging a photocurrent.   The optical communication transceiver according to claim 1, further comprising a plurality of transceivers coupled by a transparent medium.   The optical communication transceiver according to claim 1, further comprising phase synchronization means for synchronizing the transceiver with another transceiver.   The optical communication transceiver according to claim 1, wherein the first transceiver is embedded in a programmable key and the second transceiver is embedded in a programmable lock.   The optical communication transceiver according to claim 1, wherein the LED further operates as a power-on indicator when light is emitted.   The optical communication transceiver according to claim 1, wherein the LED operates as a flashlight when emitting light.   The optical communication transceiver according to claim 1, wherein the LED is embedded in an instrument.   The optical communication transceiver according to claim 1, wherein the LED is embedded in a toy. An LED coupled in series with a resistor;
A microprocessor having a first I / O pin connected to the LED and a second I / O pin coupled to the resistor;
Means for periodically driving the LED with forward bias and transmitting data by emitting light;
Means for periodically driving the LED with a reverse bias;
Means for optically discharging the LED after driving the LED with a reverse bias and receiving data by measuring the light level. An LED coupled in series with a resistor;
A microprocessor having an I / O pin connected to the LED and a ground coupled to the resistor;
Means for periodically driving the LED with forward bias by setting the I / O pin high and transmitting data by emitting light;
Means for periodically driving the LED with zero bias by setting the I / O pin to low and setting the I / O pin to an input state to optically charge the LED;
Means for measuring the level of light and receiving data. An LED coupled in series with a resistor;
A microprocessor having an I / O pin connected to the resistor and a ground coupled to the LED;
Means for periodically driving the LED with forward bias by setting the I / O pin high and transmitting data by emitting light;
Means for periodically driving the LED with zero bias by setting the I / O pin low and setting the I / O pin in an input state to optically charge the LED;
Means for measuring the level of light and receiving data. Periodically driving the LED with forward bias and transmitting data by emitting light;
Periodically driving the LED with a non-forward bias and optically changing the charge amount of the capacitance of the LED after driving the LED with a non-forward bias;
A data transmission / reception method comprising: measuring a level of the charge amount, measuring light, and receiving data.

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