Electronics

On X-plane, manipulating the Garmin 430/530 keys using the mouse can be cumbersome. Yoke keys can be assigned for these functions, but there are not enough keys on a typical yoke to make this assignment. One solution is to buy a full blown panel such as this one from Realsimgear. But it is expensive. Besides, I really don’t need the display screen because I primarily use Xplane with 3D VR. The best approach for me was to make a simple panel using rotary encoders, and make it communicate with the computer as key presses. This can be built for under $35. So that is what this article is about.

The heart of the system consists of two dual-rotary encoders with push button switches, and an Attiny 861A microcontroller. You can get Garmin-style rotary encoders from Propwashsim for $13 each. These encoders are the most expensive part of the whole system.

So, the code has two major parts. The first part is the encoder detection. The second part is the USB communication.

Encoder Detection

The entire code is available on github, but below is the encoder detection part:

tmp = getinput(AorB,pin0);      /* Check Connection 0 */
if (tmp != *state0){            /* If Connection 0 has changed state */
    _delay_ms(delay);           /* Wait for the debounce time */
    tmp = getinput(AorB,pin0);  /* Recheck the Connection 0 state */
    if (*state0 != tmp){        /* If the changed state of Connection 0 is still valid, we can move on to Connection 1 */
        *state0 = tmp;          /* Update the state of Connection 0 */
        tmp = getinput(AorB,pin1); /* Check the state of Connection 1 */
        while (tmp == *state1){  /* Wait for Connection 1 to change state */
            tmp = getinput(AorB,pin1);}
            *state1 = tmp;             /* Update the state of Connection 1 */

The code checks for a change in state (0 or 1) of one of the encoder lines (pin0). A switch debounce delay is used to make sure that it is a persistent change. Then we check the second encoder line (pin1) and wait for that to change. When that change is detected, we can conclude that a one-click turn has occurred. The process is repeated in the reverse direction to detect a turn in the opposite direction.

We have four encoders (each knob has an inner and outer knobs). Additionally, there is also a push-button function on each knob. This makes a total of ten different sensing functions: left/right on each of the four encoders, plus two push button switches. Each one of these functions can be mapped to a key press. In my case, I decided to use A, B, C, D, E and a, b, c, d and e for all ten representations.

USB

Attiny 861A does not have a built-in USB interface. But it is relatively easy to implement this function using the V-USB library. This library mimics the USB hardware function at the software level. The penalty of course is the size of the code (which will inevitably be larger) and the possible reliability issues. But for a simple application like this where the USB device is simply mimicking a keyboard, this should not be a problem. See my previous post on this topic.

Schematic

The circuit is relatively simple, though a bit crowded due to the number of wires running between the encoders and the board. I used all surface-mount components except the Attiny861A, which I prefer to have it in a DIP package with an IC socket because it allows me to unplug the chip and flash the program on a separate programming board.

Clock

The CPU clock is the most critical aspect of USB signalling. This clock has very tight tolerances, and needs to be one of several specified clock rates (12 MHz, 16 MHz or 16.5 MHz). Using an external crystal oscillator is the best way to ensure accuracy. I don’t know about you, but I don’t have these specific crystals laying around in my component collection. So I would prefer to use the chip’s internal oscillator (known as the RC oscillator). Among the three clock speeds, 16.5 MHz is the most forgiving, making it possible to use the internal RC oscillator. The RC oscillator nominally runs at 8 MHz. By enabling the PLL in the fuse bytes, this can be multiplied 8-fold to 64 MHz. However, the 64 MHz only goes to the peripheral devices. It gets divided by 4, and becomes the CPU clock at a nominal rate of 16 MHz.

The 8 MHz RC oscillator can be adjusted by writing a value into the OSCCAL register. Increasing numbers increase the clock rate. Therefore, it is possible to tune the oscillator to get 16.5 MHz.

How to measure the CPU clock

/* Created: 6/20/2020 6:33:18 PM
 * Author : Andrew Sarangan  */ 

#include <avr/io.h>

/* Fuse bits LOW.CKDIV8 should be unset*/ 
/* Fuse bits CKSEL(3:0) should be set for 64 MHz, which is 0001*/
/* This fuse is combined with SUT, so the byte is called LOW.SUT_CKSEL */
/* The waveform on 0C1B will be F_CPU/4/256/2. */
/* At 16.5 MHz, the waveform will be 8056.6 Hz */

int main(void)
{
	OSCCAL = 82;  /* This adjusts the RC oscillator frequency */
	DDRB |= (1 << PB4);    /* Use the OC1B function of the PB4 pin */
	TCCR1 |= (1 << CS10) | ( 1 << CS11); /* Divide Timer1 by 4 */
	GTCCR |= (1 << COM1B0); /*Sets the OC1B toggle operation when comparison is successful */
	OCR1B = 0;	/* This is the value to compare with Timer1 */
          
	while (1){
         ;
	}
}

By flashing this program into the chip, the output on pin #3 can be used to measure the CPU clock. When the CPU clock is 16.5 MHz, the measured pulse rate on pin #4 will be 8056.6 Hz. By trial and error, the value needed to reach 16.5 MHz was found to be OSCCAL = 82.

Voltage Levels

USB power line is 5V, but the D+ and D- lines have to be around 3.3V. This voltage conversion can be done using one of several ways:

• Use a voltage regulator to drop 5V to 3.3V, and run the chip on 3.3V.
• Alternatively, instead of a voltage regulator, a diode (or two) in series can be used to drop the 5V supply down to either 4.3V or 3.6V. Although these are not exactly 3.3V, the USB will still detect the signals correctly.
• We can also run the chip at 5V, and use two 3.3V Zener diodes (small signal diodes, not the regulating type) on the D+ and D- lines to pull the voltage down. This is the least preferred way because the Zener diodes can significantly affect the impedance and the transient response.

Test Results

Clock measurements were made using the Pokitmeter, and whether or not the USB connection worked was determined by connecting to a Windows 10 desktop computer. I did not evaluate the accuracy of the frequency measurement. Your results may vary depending on hardware, operating system, component tolerances and environmental conditions.

Circuit	Voltage	OSCCAL	Speed	Works?
Fig 1	3.3V	81	16.440 MHz	No
Fig 1	3.3V	82	16.517 MHz	Yes
Fig 1	3.3V	85	16.826 MHz	Yes
Fig 1	3.3V	86	16.985 MHz	Yes
Fig 1	3.3V	87	17.148 MHz	No
Fig 2	4.3V	81	16.328 MHz	No
Fig 2	4.3V	82	16.516 MHz	Yes
Fig 2	4.3V	86	16.905 MHz	Yes
Fig 2	4.3V	87	17.067 MHz	No
Fig 3	3.55V	81	16.421 MHz	No
Fig 3	3.55V	82	16.496 MHz	Yes
Fig 3	3.55V	86	16.960 MHz	Yes
Fig 3	3.55V	87	17.148 MHz	No
Fig 4	5V	83	16.611 MHz	No

Conclusions

The V-USB clock frequency and the voltage levels are surprisingly forgiving. At the 16.5MHz mode, the frequency can be as high as 17MHz (which is almost 3%), but it cannot be lower than 16.5 MHz. The corresponding OSCCAL range was about 82 to 86. The voltages on the data line has to be between 3.3V and 4.3V. Using Zener diodes on the data lines did not work for me, even though I verified that the Zeners were correctly clamping the voltage to 3.5V. Presumably, the switching characteristics of the Zeners were inadequate.

Normally, a bridge rectifier chip is used to convert AC to a pulsing DC. However, it can also be used to make a DC circuit insensitive to input polarity. This is really handy because I have destroyed more circuits than I like to remember because I accidentally switched the polarity of the power supply.

The downside is that the input voltage has to be about 1.4V higher than the required supply voltage. The power dissipated in the bridge rectifier is 1.4 times the current. This may not be a good solution in low voltage circuits with less head room, but for most normal applications, it is a simple way to protect the circuit from inadvertent polarity changes.