Implementation of DDS signal generation on an ATtiny85

I have started learning electronics again starting with the fundamentals, and had been looking at creating my own function generator using a 555 timer. At the right time, and more versatile, there was a attiny85 project to create simple signal generator on Hackaday. It uses ATtiny85's internal 64MHz PLL and a method named DDS (Direct Digital Synthesis) to create eight different waveforms in frequency range from 1Hz to 5kHz.

I didn't have the same I/O parts as described in David Johnson-Davies’s Tiny Function Generator blog post, so I first tried a "blind build" with a rotary encoder+switch part I already had. I built the circuit in the schematic, including the output filter (no LCD), and it didn't really work. So instead I started with each component in turn and the ATTiny85, of course.

After proving out the sub-components like rotary encoder and LCD, and demonstrating working code for them, I had to redraw the waveform characters for the 5x8 block characters on the HD44780. I converted to storing the custom characters in PROGMEM since leaving in code RAM was consuming too much memory. I also had to come up with a bank-switching scheme, since HD44780 only allows eight user-defined characters in its RAM memory at a time.

Maybe I'll be able to prove what OP said about the circuit's output passive filter characteristics when I get to those in my The Art of Electronics book that I am going through.

NOTE: Original source schematic had 6.8k resistors and 1.5 nF caps on the filter, and that's what I built originally.

• Arduino code [gist] [this repo]

• ATtiny85 PDIP-8
• rotary encoder with switch (such as KY-040 module style)
• I2C-capable LCD
  • e.g. i2c / SPI character LCD backpack and a Standard HH44870 16x2 Character LCD
• Arduino Uno, connected through USB to Arduino host computer
• 10 uF electrolytic cap
• half-size solderless breadboard with power rails to place ATTiny85 8-pin DIP onto
• a couple smaller 10-11 row mini-breadboards are handy
• a couple through-hole LEDs and 330Ω current-limiting resistors
• about two dozen jumper wires, small alligator-clip jumps
• 1k and 10k resistors
• powered USB hub to connect Arduino through to computer, and a computer such as one running macOS

Filter components

• 4.7k resistor X 2
• 10k resistor X 2
• 4.7 nF filter cap X 2
• 10 uF electrolytic cap
• 0.1 uF cap

Tools

• tiny needle nose pliers helps to insert wires into solderless breadboards
• tiny Philips head screwdriver for adjusting and wiring terminals on i2c backpack

Wiring up an Arduino UNO to flash an ATtiny is pretty quick and easy

• Arduino Uno, connected through USB to Arduino host computer
• 10 uF electrolytic cap
• half-size solderless breadboard with power rails to place ATTiny85 8-pin DIP onto
• couple through-hole LEDs and 330Ω current-limiting resistors
• about a dozen jumper wires

First flash the Uno board with ArduinoISP sketch. There is a built-in "example" sketch to achieve this: File -> Examples -> 11.ArduinoISP -> ArduinoISP

Now to flash the ATtiny itself, you'll need board support within Arduino app. Arduino "board" support for ATtinys is available from: https://github.com/SpenceKonde/ATTinyCore

Once that is installed, you can configure it for flashing our ATtiny85.

ATTiny Core settings in Arduino, using "Arduino as ISP" as "Programmer" (in Tools menu in Arduino app).

Run "Burn bootloader" before uploading sketch the first time (only needs to be done once).

• Board: "ATtiny25/45/85
• Chip: "ATtiny85"
• Clock: "8 MHz (internal)
• B.O.D.: "B.O.D. Disabled"
• LTO (1.6.11+ only): "Disabled"
• Timer 1 Clock: "CPU"

Now these settings should work to flash your development sketches onto ATtiny.

Found that I had a 16x2 character LCD display (HD44780 type) with an I2C backpack, and wanted to test it. Used TinyWireM and TinyLiquidCrystal Arduino libraries.

• Arduino test code [gist] [this repo]

The rotary encoder I had was quite a different topology than OP. I had quite a bit of trial-and error and troubleshooting to get to a robust, interrupt-driven code implementation in Arduino.

I use an Addicore Rotary Encoder with Push Switch which is an unbadged KY-040 rotary encoder module with switch AFAICT.

This version does not pull down the active-low R̅E̅S̅E̅T̅ all the way to logic zero, but instead uses a 1:10 voltage divider (and the ADCO available on pin 1) to detect a button press. The code is able to read ADC voltage and use that to determine whether input voltage falls below 10/11 Vcc (which occurs when button in rotary coder is pressed).

• Arduino test code [gist] [this repo]

BTW, here is one way to get the "combining overline" text that you see on active-low signal/pin names (in python).

print('R'+u'\u0305' + 'E'+u'\u0305' + 'S'+u'\u0305' + 'E'+u'\u0305' + 'T'+u'\u0305')

breadboard mockup