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ATtiny2313 Applications of
AVR single chip microcontrollers AT90S, ATtiny, ATmega and ATxmega
Magic circle with ATtiny2313 and LEDs

Magic circle with ATtiny2313

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An application of the controller with 13 LEDs with PWM driven intensity, preferably as a gift or displaying in the window.
  1. Properties
  2. Hardware
  3. Mounting
  4. Software

1. Properties

Magic circle Switching LEDs on and off is simple, everybody can program that using the CBI and SBI instructions in assembler (or in whatever language you prefer).

In that case the switching on works with 256 stages of increasing intensity, the switching off with the same number of stages with decreasing intensity. Call that soft on and off. Double speed in that case means reducing the number of stages down to 128, very fast speed means 64 stages. To not getting bored by the ever same speed, you can mix those speeds a little bit, so nobody goes to sleep.

The intensity regulation is performed with four PWM channels in the ATtiny2313, where the four PWM output pins OCR0A, OCR0B, OCR1A and OCR1B of the two timer/counters TC0 and TC1 toggle. Every output pin drives between two and four LED cathodes. This brings the output pins to their electical limits and active-low voltages are at 0.88 V (with four LEDs actively driven). As each anode output pin has to drive only one single LED, their active-high voltage is by 0.3 V below the operating voltage.

The display sequences are stored in a table in the flash memory, can be adjusted to your personal taste, so each device gift has an individual design and is unique.

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2. Hardware

2.1 Controller part

Schematic controller This is the whole boring scheme. Each LED is driven with a resistor (in this case with an operating voltage of 3.3 V with 33 Ω. On average a LED current of 7.2 mA (between 5.7 and 9.2 mA) has been measured. The voltage of the LED at this current is roughly 2.0 V (with 5 mm standard LEDs).

Attached is a ISP6 interface that allows to program the device within the system. Unfortunately my ancient AVR-ISP-MkII does not work at 3.3 V operating voltage. Either it is defective or programming voltage too low. Another programming device worked fine. If you encounter similar problems, switch off the 33 Ω with three jumpers during programming.

2.2 Power supply

Schematic power supply The power supply was built with a 2 VA transfor4mer with 2 * 7,5 V.

As regulator a LM317 was used, which is adjusted with a 10-speed potentiometer of 1 kΩ to 3.3 V output voltage. Of course you can use a 3.3 V regulator instead if you have one.

If a 5 V power supply shall be used the resistors driving the LEDs have to be increased to 180 or 220 Ω.

The LM317 generates at maximum 600 mA heat, so either you can use no or only a small heat sink. At elevated LED currents a 20°C/W heat sink is sufficient.

Power supply voltages These are the voltages with a consumption of 130 mA.

Power supply voltages, no load All capacitors are specified with 35 V, because the transformer produces a very high voltage when no load is attached (compensation of the inner resistance of 22,7 Ω per coil). Without a load the 7,5 V coils produces more than 16 V after rectification. Especially tantal capacitors do not survive that over-voltage.

The software for calculating the power supply voltages can be found here

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3. Mounting

3.1 Controller part

Components of the processor part Mounting the controller part is on a 40*40 mm breadboard. The LED cathode and anode connections are on soldering pins of 1 mm. If you want to have a plug instead use an 18- or 20-pin IDC connector instead.

Controller board That is how the board looks like after soldering the LEDs.

3.2 Mounting the power supply

Components of the power supply The power supply components fit to a 50*60 mm breadboard. Especially the 230/110 V side needs some attention: the mounting screws shall not have contact with the fuse connections.

Power supply That is how the power supply looks like. The type plate of the transformer is the other way around, presumably that is why it was so cheap.

3.3 Mounting into a box

Box mounting The whole electronics fits into a small 120-by-60-by-40 mm plastic box, which is equipped with a small type plate (in Open-Office-Graphic format), and four rubber foots and finally delivered with a user manual in PDF or in Open-Office-Writer format to the receiver.

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4. Software

4.1 Structure of the software

During init all three ports A, B and D that are used to drive the LEDs are written to ones. The direction ports of the output pins OC0A, OC0B, OC1A and OC1B are set, all anode output pins are cleared (all LEDs off). The pointer Z is set to the beginning of the sequence table. Both timers are started with a precaler of 8 and all OC outputs are set to toggle (0 on the beginning of the PWM cycle, toggle when reaching the compare value).

Following init, the software runs interrupt-driven. For this, the CompareA interrupt of timer/counter TC0 is used, which is triggered after each complete PWM phase. The interrupt-service-routine then sets the compare values of all four PWM channels to the value in the register rPwm. This register is then either increased (flag bFall = 0) or decreased (flag bFall = 1). If the flag bFast is one increase and decrease are by one additional step, if the flag bVeryFast is one, two additional steps are performed.

If the register rPwm overflows (bFall=0) and if the flag bUpOnly is set, the rPwm restarts at zero. If bUpOnly=0 then bFall is set.

If the counter rPwm reaches zero (bUpOnly=0 and bFall=1) or if it overflows (bUpOnly=1), the flag bStart will be set. This flag is processed outside the interrupt service routine: the next three LED anode combinations are read from the table and are written to the three direction ports. All LEDs that should be on in the next period have their direction bit set and the portpin gets high.

The processing steps are described in the next chapter.

4.2 Processing steps

All processing steps are laid down in a table. This usually holds four data bytes:
  1. First byte: Flags: The flags can be normal (0), upwards only (bit 0 = 1), double speed (bit 1 = 1) or fourfold speed (bit 1 and 2 = 1).
  2. Second byte: The direction bits for port D (with bit 6 always set).
  3. Third byte: The direction bits for port B (with bits 2, 3 and 4 always set).
  4. Fourth byte: The direction bits for port A.
If the flag byte has the following values, the process is controlled and only one following byte is read: To ease the construction of the process table, the file defines bit combinations, that can be used to write data to the table. For example the values for no LED on and for all LEDs on are:

.equ X0=1843200 ; 00.1C.20.00
.equ Xall=67075840 ; 03.FF.7F.00


.set cX = X1
.db Byte1(cX),Byte2(cX),Byte3(cX),Byte4(cX)
.set cX = X2
.db Byte1(cX),Byte2(cX),Byte3(cX),Byte4(cX)

the LEDs one and two are switched on.

.set cX = X1_2
.db Byte1(cX),Byte2(cX),Byte3(cX),Byte4(cX)

This turns both LEDs on.

The flags bUpOnly, bFast and bVeryFast are simply added to this constant, e.g. very fast on/off of LED 2:

.set cX = cVeryFast+X2
.db Byte1(cX),Byte2(cX),Byte3(cX),Byte4(cX)

By using the repetition control up to 256 repetitions can be encoded without having to copy the sequence n times. This repeats the sequence LED 1, LED 2 and LED 3 for four times.

.db cRepeat1,4 ; Start outer repetion, count = 4
	.set cX = X1
	.db Byte1(cX),Byte2(cX),Byte3(cX),Byte4(cX)
	.set cX = X2
	.db Byte1(cX),Byte2(cX),Byte3(cX),Byte4(cX)
	.set cX = X3
	.db Byte1(cX),Byte2(cX),Byte3(cX),Byte4(cX)
	.db cNext1,0 ; Repeat outer loop three times

With cRepeat1 and cRepeat2 nested repetitions can be constructed.

Spread sheet If necessary the spread sheet Tables.ods in Open-Office format allows to design and calculate combinations. The sheet Bit_Tab allows that. Just Make sure that the changed table always ends with 0xFF,0xFF, so that execution restarts.

4.3 Download

The assembler source code is here in assembler format and here in HTML format. For assembling the Include file is additionally required in the same path as the .asm file.

No fuses have to be changed.

Have fun with the soft blinker.

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