Skip to main content

ESPCLOCK2, Part 3 - Reading VCC using interrupt

In this post, I read the VCC voltage level to the ATtiny85 and save certain state values to the EEPROM if the voltage level is found to fall below a certain value. But this way of reading the VCC uses delay() and is found to interfere with I2C communication. So a better way needs to be found.

After much experimentation, I found the following code to work reliably:

volatile byte adc_countdown = 5;
volatile bool adc_ready = false;

// adc_countdown is decremented elsewhere every second
if (adc_ready) {
  adc_ready = false;
  vcc = 1125300L / ADC; // Calculate VCC (in mV); 1125300 = 1.1*1023*1000
  adc_countdown = 5;
else if (!bit_is_set(ADCSRA, ADSC) && adc_countdown == 0) {
  ADMUX = bit(MUX3) | bit(MUX2); // Measure VCC using internal bandgap as reference
  ADCSRA = bit(ADEN) | bit(ADSC) | bit(ADIE) | bit(ADPS2) | bit(ADPS1) | bit(ADPS0); // Start conversion

ISR(ADC_vect) {
  adc_ready = true;

Initially, the ADC register is read in the ISR. But even that takes long enough to interfere with I2C. So a flag adc_ready is set in the ISR, and the actual reading takes place in loop().

The counter adc_countdown makes sure we don't run the ADC too frequently to conserve power (currently set to every 5 seconds).


Popular posts from this blog

Update: Line adapter for Ozito Blade Trimmer

Update (Dec 2021): If you access to a 3D printer, I would now recommend this solution , which makes it super easy to replace the trimmer line. I have been using it for a few months now with zero issue.

Filament Joiner Part 2 (With Display and Knob)

Thanks to the current corona-virus crisis, the parts I ordered for the filament joiner project were taking forever to arrive. But now that they have finally arrived, I can put them to good use. These were the parts ordered: 0.96" OLED display SSD1306 Rotary switch encoder KY-040 Here is the final circuit diagram: The OLED display is connected to the SCK and SDA pins of the Nano (A2 and A3 respectively), and powered by 5V and GND. The rotary switch encoder is connected as follows: VCC => 5V GND = > GND CLK => D9 DT => D8 SW => D2 My prototype board now looks like this: The updated code for driving the knob and display is available in  heater-with-display.ino in the Github repository . We now have a fairly compact (about 7cm x 5cm) and independent filament joiner (no need to connect to PC) that is driven solely by a 12V power supply. Here's how to use it to join printer filaments. More usage details in my previous post .

Adding "Stereo Mixer" to Windows 7 with Conexant sound card

This procedure worked for my laptop (Thinkpad E530) with a Conexant 20671 sound card, but I suspect it will work for other sound cards in the Conexant family. I was playing with CamStudio to do a video capture of a Flash-based cartoon so that I can put it on the WDTV media player and play it on the big screen in the living room for my kids. The video capture worked brilliantly, but to do a sound capture, I needed to do some hacking. Apparently, there was this recording device called "Stereo Mixer" that was pretty standard in the Windows XP days. This allowed you to capture whatever was played to the speaker in all its digital glory. Then under pressure from various organizations on the dark side of the force, Microsoft and soundcard makers starting disabling this wonderful feature from Windows Vista onwards. So after much Googling around, I found out that for most sound cards, the hardware feature is still there, just not enabled on the software side. Unfortunately, to