Dan White dan.white@valpo.edu

Two-probe device to find low resistance paths with audible output.

Progressive design build and test posts. shortsquawker-1

1. Features

Two probe pins

Button:

  • Hold: zero to current value

  • Press: activate / deactivate alternating ref-meas tone

    • Short press captures current value as reference

    • Tone alternates between live and reference. Or two tones at once?? TBD

Tone:

  • Pitch is proportional to resistance

2. Design

short squawker complete

2.1. Pin assignments

attiny pinout

  • PB4: Probe +

  • PB3: Probe −

  • PB1: Speaker

  • PB0: unused (could be speaker −)

  • PB2: Button. Active-low, internal pull-up

2.2. Configuration

2.2.1. Clocks

  • 8 MHz internal RC oscillator

  • No prescaler

  • → lfuse = 0xE2

2.2.2. Pins

2.2.3. ADC setup

  • 1.1 V reference

    • REFS[2:0] = 0b010, Table 17-3, p.134

    • ADMUX |= 0x80 (note random bit order in reg §17.13.1, p.134)

  • Unipolar differential with 20x gain, Table 17-4, p.135

    • MUX[3:0] =

    • 0101 PB4 - PB4, ADC 2-2 (self offset)

    • 0111 PB4 - PB3, ADC 2-3

    • ADMUX |= 0x05 or 0x07

  • ADC clock prescaler

    • if f_cpu = 8 MHz, prescale /64 gives fADC=125 kHz

    • ADCPS[2:0] = 0b110

    • ADCSRA |= 0x06

  • Disable digital input buffers. §17.13.5, p.138

    • ADC[3:2]D = 1

    • DIDR0 = 0x18

This setup gives: 1.1 Vref / 20x / 210 = 53.7 uV per LSb

10.74 mA test current yields 5mOhm per LSb

2.2.4. Timer0 setup

Interrupt every timer overflow as a system tick rate of: 8 MHz / 8 / 256 = 3.906 kHz

This is the PWM update rate, or output sample rate.

This tick updates a digitally-controlled oscillator (DCO), whose phase value is then converted to PWM-generated amplitude of sin() or tri() using a lookup table.

  • Prescale by 8

    • TCCR0B = 0x02

  • Enable overflow interrupt

    • **TIMSK |= (1 << TOIE0)

2.2.5. Timer1

Timer1 generates fast PWM signal.

OC1A

  • COM1A[1:0] = 0b01

Output frequency range:

  • max 4 kHz / 250 μs

  • min ???

Max resistance

  • 1.024 Ω ???

fout = 4 kHz / (Rtest / Rmax)

5mΩ resistance LSb increment

Timer compare, reset count, toggle output pin.