Electronics notes / Inputs and outputs

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High side versus low side switching/driving

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

On ADCs and DACs

ADC

An Analog-Digital Converter (ADC) takes a (voltage) signal and turns it into numbers to be digitally consumed.

Well, most ADCs do voltage sensing. A few have extra circuitry to be current-input ADCs (which mostly just means a resistor setup, but probably better specced than you can easily make), many of them for specific applications.

DAC

On the cheap: Resistor ladder

Basic idea: If you can change many (e.g. often 8 on microcontrollers) digital pins at the same time, then putting a resistor ladder on them means each contributes a different voltage level, and you can make 2^amt voltage levels (256 for 8 pins).

E.g. the Covox Speech Thing was basically just a bunch of resistors on a parallel port. adopted by some games and music trackers.

Limitations:

• you still need to write out new data with strict regularity

which is what interrupts are good at, but it will occupy a bunch of CPU power

The Disney Sound Source solved this by being buffered (but fixed the output rate to a relatively poor 7kHz)

• the resistors' voltage contribution won't be perfect to contribute exactly what they should.

making equal steps (to produce undistorted signals) takes either precision resistors or a bunch of measuring and matching beforehand.

the more bits you have, the harder it gets - 8 is quite doable, but it starts getting impractical around 12 bits

• more than 8 bit is rare

it's often the max you can change at the same time (single uC port)

using multiple ports is hard to get quite right, though may be acceptable at lower frequencies

Example:

• http://www.radanpro.com/Radan2400/mikrokontroleri/Jesper%27s%20AVR%20pages%20-%20MiniDDS.htm

• https://en.wikipedia.org/wiki/Covox_Speech_Thing

On the cheap: Fast PWM

Basic idea: If we can PWM much faster than audible (by preferably two orders of magnitude - which basically asks "if we have dedicated PWM circuit in the IC"), then the duty cycle is controlled precisely enough that putting it through a lowpass means the result is stable enough (being an average of that duty cycle).

These work fine, though are limited frequency-wise. They're can work pretty decently in audible frequencies.

Limitations:

• the lowpass is fixed

so it has a range where it works best, which is part of your design

which is why this can't be a more general-purpose DDS

• without dedicated PWM circuitry the timing issues would make this a horribly task, particularly if you also want to do other things in the microcontroller

• even then we need to update the PWM with strict regularity

this is what interrupts are good at. It just occupies the CPU.

• i.e. even when it's not CPU-intensive it's still timing-critical

Expect everything else to be secondary to this.

• multiple channels may be possible (varies with IC) but may have to be related

which is perfectly fine if you e.g. wanted different types of wave at the same rate

Example:

• http://digital-wizard.net/mcu_interfacing/sine_wave_generation
• https://web.csulb.edu/~hill/ee470/Lab%202d%20-%20Sine_Wave_Generator.pdf

• Raspberry Pi's onboard sound is actually PWM pins, plus a filter and buffer (exact details vary)

See also

Digital sample capture, storage, reproduction