The AtTiny85 packs a lot of features packed into the AtTiny85 pin dual in line package (DIP) chip. There are a number of features that I like about the AtTiny85 micro-controller. The small chip size is great with 8 pins available for fun.
Two of the pins are used to power the chip. One pin for 5 volts or 3.3 volts direct current power and one for ground. That leaves 6 pins for fun stuff. One of the six pins is a reset pin, which can be made into another pin for fun by setting the fuse to no reset. If you do this, then you can only program the chip with high voltage programming using a special programmer. An Atmel Dragon programming board can do high voltage programming and set the fuses. I used the Dragon to set the no reset pin, so I can use it for fun. I like to use the internal clock set to 8MHz and that is done setting the fuses too. Using the internal clock is nice because I don’t need an external crystal, which adds cost and takes up two pins.
Six pins are great for LED projects, because 6 LEDs can be flashing away all at once connected to the little chip. There are 3 of the 6 that are pulse width modulation (PWM) pins. These 3 pins can be used to drive an red-green-blue (RGB) LED and have 3 single LEDs running at the same time for a total of 4 LEDs. I want to talk about the PWM pins, because there are many diagrams on the Internet that show 2 or 4 PWM pins, which is not true in either case. The data sheet from Atmel shows pin 2 having and inverted oscillator OC1B for PWM, but this pin does not work as a PWM pin. I think, I read there was a problem with this pin from day one. I tested this pin and it does not do PWM. Pins 3, 5, & 6 do have PWM, which I also tested and they work, so you can use an RGB LED and enjoy it. Because of all the confusion with what pins have PWM or not, many of the Arduino hardware libraries for the AtTiny85 are wrong. You will have to read the pins_arduino.h files to make sure the correct pins have PWM in the code. I think, I made a code change in this file to fix the problem or I found the correct library to use. (**I will update this later to share the details**)
The memory is large enough for a cool programs to be stored. The flash for programming is 8 kilobytes (kb) in size. The static random access memory (SRAM) is 512 bytes in size.
I purchased two 630F 2.5V(max) super caps recently. They do not perform the way I expected. I charged my caps at about 1A until they reached about 2.4VDC, so I would not over charge them. I left them overnight, when I check the voltage in the morning they were both at 1.3VDC. I did not expect this loss.
So, I decided to test this in the morning and I charged them again to 2.4V. After charging I placed a voltage meter on one cap, and the voltage starting bleeding off and in about 15 minutes it was down to 2V. This was like a bleeder resistor was on the cap. I had nothing but the meter on the cap and only for a few seconds then removed.
The point is expect this lower steady state voltage after charging of about HALF of the maximum rating. I paid $20 for each cap plus shipping. Also, the 630F capacity is only about 0.6 Watt-Hours, so pretty low capacity.
Super caps should be used with some care, but there are NOT a battery.
I have added more features to my tablet. The batteries are in parallel, so the load is shared from the screen and the RPi. A switching 12VDC to 5VDC regulator replaced a very warm LM338 5A linear regulator. The switcher is from a cigarette lighter to USB car charger. It runs very cool in the case. A battery charger was added using a wall wart 12VDC @500mA. I connected an extension cord to the wall wart and fed it outside the case, because I don`t want to carry another charger around like cell phones and iPads have. Just a cord and a wall outlet.
The cigarette lighter plug is active and has a 7.5A fuse just in case. The lighter port can be used in two ways, power supply and 12VDC charger for charging the internal batteries.
Switches have been added for the master power from the batteries to the rest of the tablet, screen power, RPi and USB hub, and internal battery charger. The switches get the user full control of th tablet.