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+5V Q1 BC556 f g e 6 D3 D4 IC1 ATTINY 2313 d DISP2 NFD-8021BS a b c f e g d dp a b f g e c dp d D1 D2 g f e d c b a dp 1 2 3 4 5 6 7 8 9 10 DISP3 NFD-5621BS a b c f e d dp a b g c dp What it is & how it works PD2 g f e d c b a dp 1 2 3 4 5 6 7 8 9 10 f e g d a b f g e c d dp c dp e d g b c dp f b a 1 2 3 4 5 6 7 8 9 10 PB3 15 14 PB2 13 “Charlieplexing” is a multiplexing technique for driving displays PB1 12 PB0 and is particularly suited for use with microcontrollers. It 11 PD6 reduces the number of outputs required to drive a given display 16 PB4 but implementing it can be a little tricky. Here is an in-depth PB5 17 18 8explanation PB6 of how it works. 19 PD4 PB7 2 PD0 By NICHOLAS VINEN T PA2/RST 1 +5V RLY1 K HE REMOTE-CONTROLLED Digi- all six digits using just nine tracks in place of the specialised IC. This tal Up/Down Timer described in and no external components (apart D2also allowed us to optimise the track August 2010 features six920mm-high from the microcontroller). The method connections, dramatically simplifying PD5 LED digits in three dual displays. we used is known as “Charlieplex- the board layout. A These are great because they 3 are bright ing”. It was originally developed by PD1 and easy to read from a distance but Christopher Malinowski in 1979 but Multiplexing unlike a Liquid Crystal Display (LCD), was popularised by Charlie Allen at Before describing how the technique they do not have an on-board control- Maxim, with the development of the works, let’s first look at how a 7-seg4 ler IC. This means that driving them MAX6950 and MAX6951 ICs. These ment LED display is normally driven. XTAL2 D5 normally involves + using external can drive five or eight 7-segment dis- This method is not restricted to this components and quite a few tracks on plays from a 16-pin ICPIEZO package. type of display – it can be used with the7PC board. We used a similarBUZZER scheme for our vacuum fluorescent displays, incanX1 8MHz PD3 we found a way to drive timer project, with a microcontroller descent displays, Nixie tubes etc. The However, 5 – XTAL1 D6 GND 10 D6: 1N4148 A a D2 D1 Charlieplexing DISP1 NFD-8021BS 20 Vdd D1 D2 470Ω CON4 K 33pF 10k C B 33pF Q2 BC546 C C CON2 E D1, D2: 1N4004 A LLED DIGITAL TIMER LED K B K A 7805 BC546, BC556 E IN C GND OUT Fig.1: the conventional multiplexing scheme for driving common anode LED displays. Each digit is individually switched on in sequence and the appropriate cathode control lines drive the wanted segments via transistors Q5-Q8. siliconchip.com.au CON1 September 2010  67 Fig.2: this shows how the same four LED displays are driven using Charlieplexing. In this case, the anodes and cathodes are driven using complementary transistor pairs. This is convenient when using a microcontroller, as their digital outputs are typically configured in this manner. main differences are the drive voltage and current. Fig.1 shows the conventional multiplexing arrangement, although we have simplified it by showing just four LEDs in each display and only four displays. However, this method can handle any number of segments or displays by adding more transistors and control lines. Of course, as the number increases, routing the tracks becomes more difficult. It works as follows: the anode control lines are active-low and only one of them is active at any given time. Typically, transistors Q1-Q4 are turned on in sequence with a 25% duty cycle. Conversely, the cathode control lines are active-high and any combination may be turned on at any given time. Let’s say we want to light LEDs 2, 4, 5 & 9. First, anode control line 1 is brought low and Q1 is turned on. At 68  Silicon Chip the same time, cathode control lines 2 & 4 are switched high. As a result, LEDs 1-4 are driven at the anode but only LEDs 2 & 4 are driven at their cathodes, so only these two LEDs light. Before Q1 is turned off again, all the cathode control lines are brought low (off). Q2 then switches on and cathode control line 1 is switched high. In this case, LED5-8 are driven at their anodes but only LED5 is driven at its cathode and so only LED5 lights. Finally, LED9 is switched on by turning Q3 on (after Q2 is turned off) and taking cathode control line 1 high. All the cathode control lines (and thus the cathode drivers) then remain off while Q4 is turned on, so that no further LEDs light. Duty cycle In this circuit, any LED can only be on 25% of the time because the driver transistors are switched in sequence. So in reality, when multiplexed LEDs are alight, they are actually flashing. However, by making them flash rapidly enough, they appear to be lit continuously due to persistence of vision (ie, our eyes cannot react rapidly enough to detect the flashing). As a result, we just see the lit segments. It also follows that the LEDs are 25% as bright as they would be if switched on permanently, assuming equal currents. In practice, this is compensated for by using brighter LEDs or by driving them at higher peak currents. Most LEDs can handle much higher currents for brief periods than they can on a continuous basis (ie, at DC), as the ultimate limit is their dissipation rating. If they are only switched on a portion of the time, the average dissipation is a fraction of the instantaneous dissipation (in this case, 25%). This multiplexing scheme is widely used and it works well but it has dissiliconchip.com.au advantages. The main problem is the number of control lines necessary. It is the sum of the number of segments and the number of digits. For last month’s Digital Timer with six 7-segment digits (which have eight LEDs each, including the decimal point), this would mean 6 + 8 = 14 control lines. It would also require 14 transistors and 22 resistors. This could be reduced by driving the cathodes directly from the microcontroller outputs but if this is done, it’s difficult to make the segment brightness consistent between digits due to internal resistances in the microcontroller’s power supply. By using Charlieplexing, we managed to drive the displays using nine pins and no additional components. Charlieplexing Now take a look at the “Charlieplexing” arrangement shown in Fig.2. This shows the same LED displays as before but with the new driving arrangement. There are actually more transistors, five PNP and five NPN, but they are arranged as complementary pairs. This is handy because microcontroller digital outputs have a similar configuration. If the transistors in a complementary pair are individually controlled, as they are in a micro, we have what is known as a “Tri-state” output (a term coined by National Semiconductor). If one transistor is on and the other is off, then the output is either active high or active low (ie, pull-up or pull-down). If both transistors are off, then the output is “high-impedance”, ie, not driven. Switching both transistors on creates a short circuit between the supply rails so this condition is prevented. Transistor pairs 1-4 drive one common anode and between one and four cathodes. It doesn’t matter which cathodes are driven by which pair. However, the common anode of each display is driven by a different pair than for its cathodes. As before, one anode is brought high at any given time. Transistor Q1 drives the first display’s common anode, Q3 the second, and so on. When Q1 is switched on, the first display’s four cathodes are controlled by Q4, Q6, Q8 & Q10. Similarly, when Q3 is switched on, the second display’s four cathodes are controlled by Q6, Q8, Q10 & Q2. Now, say we want to turn on the same LEDs as in our last example (ie, LEDs 2, 4, 5 & 9). We set pair 1 to acsiliconchip.com.au tive high, pairs 2 and 4 to active low and pair 3 to high impedance. That means that transistors Q1, Q4 and Q8 will be switched on while the rest will remain off. As a result, LED2 and LED4 will both be forward biased so they will light up. Now consider LED8 in this state. It is also being driven at both ends – its anode by Q4 and its cathode by Q1. However because its anode is pulled low and its cathode high, it is reverse biased. Since LEDs act as diodes, no current will flow and so LED8 will remain off. This is the trick which allows Charlieplexing to control the same number of display segments with less control lines. As before, to light the other segments, the microcontroller cycles through the anodes and turns on the appropriate cathodes. In the case of the MAX6590/1 ICs, not only are the transistor pairs internal to the IC but the cathode transistors have an adjustable current limit, so no additional parts are required. You just need the IC, the four digits and five tracks. Since the order of the anodes and cathodes are not important, they can be re-mapped in the software – we can route those five tracks whichever way is most convenient. As demonstrated in last month’s timer project, similar results can be obtained with a microcontroller. The catch is that driving the display requires a fair bit of processing power as it is necessary to cycle through the segments one at a time and light them (or not) individually. This is because we must rely on the internal resistance in their output transistors to limit the current without additional resistors. It is possible to add one resistor per cathode set to the circuit to solve this problem although brightness matching between digits can then become an issue, depending on the current at which the segments are being driven. However, either technique is valid and can provide simpler circuitry than the traditional multiplexing method. Because Charlieplexing is software intensive, it is best suited to applications where the microcontroller does not have a lot of work to do besides driving the display. This is the case with our Digital Up/Down Timer and there are many other applications which can take advantage of “Charlieplexing” to simplify circuit design SC and PC board layout. Helping to put you in Control Control Equipment Freetronics TwentyTen Based on the Arduino Duemilanove but with improvements and updates It's 100% Arduino compatible. 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Commonly used to measure shaft speed. $27.50+GST Ph: 03 9782 5882 New, Easier to Use Website www.oceancontrols.com.au September 2010  69