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AVR Single chip controllers AT90S, ATtiny, ATmega and ATxmega
Servomotor controller with an ATtiny24

Model railroad crossing gate with an ATtiny24 and a servomotor

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If you are

addicted to railway models, AND you are
not that rich to invest hundreds of dollars to just open and close model railway gates smoothly (with this here available for less than 10$ or € all in all), OR if you are
a complete newbie in AVR assembly programming and want to learn 16-by-8 bit binary multiplication and 16-by-8 bit division by practice, OR if you are
bored by C coding, because it gives you no real control over the hardware of the controller and you do not find a library that does exactly what you need, OR if you are
disappointed about C libraries because they reduce you to a dump caller of routines that you neither understand nor have been written by yourself nor you have a chance to fully understand nor have any control over, OR if you are
completely on a mass drug called Arduino, your projects were so far solely on an ATmega328 and you don't know something else beyond that, OR if you are
searching for new worlds that open up your horizon of deeper understanding opportunities and limits of modern technology,

this is a small and simple to build project that fits your needs. The whole software is documented here, so you can even understand how it works. If necessary you can inspect it instruction word by instruction word, no hidden things or intransparent secrets here. And it provides you with a very flexible and easy to adjust piece of lean, stand-alone hardware.

1 Properties

Here is a servomoter controller for model railroad crossing gates with the following properties:

opens and closes the gate with two push buttons,
allows you to adjust the upper and lower positioning with two trim potentiometers,
allows you to adjust the speed of movement with a separate trim potentiometer,
Display of the current state with a red/green duo-LED:
- Gate fully open: permanently green,
- Gate fully closed: permanently red,
- Gate opens or closes: LED blinks red,
wide operating voltage range, 2.7 to 5.5 Volt, depending from your servomotor,
provides open source assembler code, available for download.

Hardware	Mounting	Software	Pictures/videos

1 Hardware

1.1 Schematic

The schematic is simple. The controller ATtiny24 controls it all. Three 10k trim potentiometers are attached to the ADC0, ADC1 and ADC2 pins. The red/green duo LED is attached to the port pins PB0 (anode red, cathode green) and PB1 (cathode red, anode green). The two push buttons are on the INT0 and PCINT7 input pins. The pulse-width-modulated signal for the servomotor is generated on output OC1A. If programming of the ATtiny24 shall be performed a 6-pin In-System-Programmer can be attached via the standard ISP6 plug.

1.2 Electrical functions

All electrical functions are performed by the micro-controller ATtiny24.

He runs with the internal RC clock generator of 8 MHz, divided by 8. This is the standard configuration with which the ATtiny24 is shipped.
The angle, at which the three trim potentiometers on the pins 13, 12 and 11 of the ATtiny24 are actually trimmed, is read by the analog-to-digital converter. 64 measurements are summed up for each of the three channels. Such a cycle lasts one third of a second. If the gate is in the open position, the gate follows any changes on the open trim. If in close position, the gate follows changes on the close trim.
Closing of the gate is initiated by the push button on the INT0 input pin, and if the INT0 pin is grounded on start-up.
Opening of the gate is initiated by the push button on the PCINT7 input pin and by default at start-up.
The PWM output signal that controls the position of the servomotor is generated on output pin OC1A. The pulse width of the active-high signal determines the angle, to which the servomotor moves. 50 PWM cycles are performed per second.
The micro-controller can be programmed via the ISP6 interface. Programmer pulses are received/send via the MOSI, MISO and SCK pins, programming is initiated by pulling the RESET input low. Programming does not interfere with the servomotor because the programming pulses are much faster than servomotor signals.

1.3 PWM signal for the servomotor

The PWM signal for the servomotor looks like this: the pin is on for a certain period and off for the remaining duration to 20 ms.

The active signal duration determines the position of the motor. It can be between 900 and 2,100 ľs length. The short duration brings the motor to its rightmost position, the longest to its leftmost position. 1,500 ľs is the middle position. The position of the trim potentiometer P1 varies the signal duration from 1,500 (trim = 0°) down to 800 ľs (trim = 270°) for closing the gate. Trim potentiometer P2 varies the signal duration between 1,500 (trim = 0°) and 2,200 ľs (trim = 270°).

Speed control of the motor

When opening or closing the gate the signal duration changes: when closing it gets shorter, when opening it gets longer. In both cases movement stops when the lower and upper limit is reached. The speed of the in- and decreases is determined by the trim potentiometer P3. On P3 = 0° increase and decrease lasts for seven seconds (350 PWM cycles). If P3 = 270° the whole movement lasts only one second (50 PWM cycles). Changes of P3 come only into effect when the gate is moving.

1.4 Variations of the hardware

If you need two LEDs (one for each side of the gates): just connect the second LED with its own current-limiting resistor to PB0 and PB1. Up to seven LEDs can be driven by these two outputs pins.

If you need only two, you can also connect them serially and reduce the resistor down to 100 Ω (with 5 V operating voltage). Or if you need four: connect two times two LEDs serially and with two resistors of 100 Ω to PB0 and PB1. Up to 14 LEDs can be driven in this way.

If, instead of two push buttons, you want to use only one switch that is switching to close or open the gate, use a one with a middle pin. Connect the middle pin to ground (GND) and the two other pins to the INT0 and PCINT7 input pins. That works fine, too.

Other extensions, such as receiving and decoding remote control signals on e.g. the PA3 input pin, require some additional software. Controller-internal hardware such as the 8 bit timer TC0, flash storage space, SRAM for storage or six upper registers, of which two register pairs can be used as pointers, and six lower registers are still available. Enough resources that allow for own extensions.

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2 Mounting

Mounting comes in two versions, V2 and V3. The only difference between those concerns the pins for the servo motor:

Version 2 with three 1 mm pins in 5.12 mm distance, with GND and VCC on the left and right pins,
Version 3 with three pins in 2.54 mm distance, with GND on the right pin, VCC in the mid and the PWM signal on the left pin.

The latter is for plugging in standard servomotors directly. Both versions are 50-by-40 mm.

2.1 PCB Version 2

Two bridges have to be soldered in this version.

2.2 PCB Version 3

Three bridges have to be soldered in this version.

The final product looks like that:

PCB from above

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3 Software

The software comes in two versions, too. Version 1 is the older original source from 2004, translated to english. Version 2 is more recent, makes better use of the available resources in the ATtiny24 and is my favored. The functionality is nearly the same. Both source codes are in assembler.

3.1 Source code V1

The source code can be downloaded here and can be viewed in the browser here. The following describes how it works in detail.

3.1.1 Functioning of the software V1

See the extensive further comments in the source code in an editor for detailed descriptions of the functioning. On start up of the controller the current status of the trim potentiometer is read in, the 64 values on each channel are summed up (0..65,535) and the positions are converted as follows:

Trim 1: The sum is divided by 128 and 900 is added. This yields the lower position of the gate as pulse duration for the servomotor in micro-seconds.
Trim 2: The sum value is divided by 128 and (2100 - 511) is added. This yields the open position as pulse duration in micro-seconds.
Trim 3: The sum value is divided by 256, multiplied by 24, divided by 256 and one is added. With this value the opening and closing value is changed.

The three values are written to registers.

On start up the internal hardware of the controller is set up:

The AD converter is started with a clock prescaling of 128 and the conversion complete interrupt enabled. For the three AD port pins the digital drivers are switched of to reduce power consumption.
The port pins for the up/down push buttons are configured as input pins with the pull-up resistors switched on. Both input pins trigger external interrupts (INT0 resp. PCINT0).
The two port pins for the LED are configured as outputs.
The 16 bit timer TC1 is configured as CTC (Clear Timer on Compare) with a prescaler value of 1. On compare match the TC1COMPA interrupt is executed. The output OC1A is toggled on compare match.

3.1.2 Interrupts

All further execution is interrupt-controlled.

The AD converter reads the last result in 10 bit mode and adds the result to a sum value in SRAM. Following each conversion the input MUX channel is increased, channel 0 restarts if two is exceeded. If 64 rounds are reached, the sums are copied to registers, cleared for restart and a flag is set.
The timer interrupt of TC1 first checks if the OC1A pin is high (active signal is started) or low (inactive period). Depending from that, either the duration of the short active period or the long inactive period duration is loaded to the compare port register. Both durations add up to 20,000 µ. If the signal output is low, a flag is set.
INT0 clears the upward flag, PCIN0 sets it. If the motor is inactive, the gate movement is started. If the toggle bit is cleared, the output OC1A is set to toggle.

All further actions are triggered via flags.

3.1.3 Flags

Following wake-up of the controller by interrupts (sleep mode idle) flags are consulted to check if handling is required. Following flag handling the controller is send to sleep again.

The following handling routines are triggered:

If the ADC flag is set, the conversion of the values is performed. If one of the position values has changed and if the gate is completely closed or opened the new value is written immediately for ten times to the servomotor (adjust mode).
If the timer flag is set and if gate movement is active adding resp. subtracting the delta value takes place. If the upper or lower limit is reached the gate movement flag is cleared and a post-PWM period starts. If that period is over, the toggle flag is cleared and the OC1A output will be cleared on compare match. Depending from the phase in which the motor state is, the LED is controlled.

3.2 Version 2 of the source code

A new version V2 has been posted here, wich can be viewed in HTML format here. The new version has the following improved properties:

16 bit timer TC1 now operates in fast PWM mode (mode 14), relieving the compare match A interrupt from the task of switching compare A match values between active and inactive signal periods.
The ICR port register in TC1 is now used as CTC value instead of compare match A. ICR is fixed at 20,000 µs, compare match A now works as value selection for the active signal duration. Pin control on OC1A (port pin PA6) is selected to set on bottom count and to clear on compare match A.
The PWM pulse width is now between 800 and 2,200 (previous version: 900 and 2,100). This is sufficient for available servomotors.
The duration over which the gate opens and closes (between one and seven seconds) is now independent from the selected opening and closing positions. This is achieved by dividing the range difference between the closed and opening position by the third trim potentiometer's position (7 seconds = 350 PWM cycles for trim at 0° and MSB=0, 1 second = 50 PWM cycles for trim=270° and MSB=255). This allows for a constant closure and opening time instead of speed depending from the positions. Note that in the highest speed selection (trim 3 in 270° position) this only works correct for opening angles of 31° upwards due to rounding effects.
Up- or down-movement of the motor is now controlled via the T-flag in the status register port. This decision simplifies external interrupt processing to react on push button events. Flags now are generally simplified.
ADC measurements are now organized 64 mesasurements for each channel in a row, which reliefs the ADCRDY interrupt service routine from the task of switching between channels. Channel selection now is done outside the interrupt service routine. Because only one sum and not three different sums have to be calculated, summing in SRAM is avoided, also an optimization.

The new version is much simpler in its structure, much easier to understand and considerably shorter than the previous version.

The source code includes a detailled description of the software's functioning. Open it in an editor of your choice to go deeper into that.

This is the PWM signal generated on the OC1A port pin. This is rather exact.

4. Pictures and videos

Here is a picture after the servomotor reached its closed position and the red LED is permanently on.

Gate tn24 closed

And here the gate is in its open position and the green LED is on.

Gate tn24 opened

In this video the gate moves slow from the open to the closed and back to the open position. In this video the movement is fast. If those videos do not play automatically in your browser, download them and start them locally in your favored video player.

Here two videos show a steam hammer model that uses the control device.

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