Electronics notes / Inputs and outputs

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High side versus low side switching / driving / sensing

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

Transistor output stages

Open-drain / open-collector

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

On ADCs and DACs

ADC

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

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

A Digital-Analog Converter (DAC) takes numbers and turn them into voltages.

What do DACs speak

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

That varies.

It's often one of

• I2C or SPI

a little more fiddly to use for sound because the timing is either separate or not really defined

but perfectly good for "set voltage level on output"

• I2S - good at sound

note that there are technically some dialects

• something specific
  • e.g. OPL style FM (e.g. YM3812, YMF262) want a specific style of DAC (e.g. YAC512), which sends floating-point-like values
  • TI DSPs often speak speak TI's McBSP, and so various of their ADC/DACs

There are some further tricks that can work - sometimes more for the benefit of the sound-sending side.

because any way to get regular sample output done for you is something you don't have to guarantee yourself.

Consider also:

• Pulse Density Modulation in general

• Abusing a serial port at a high rate, see e.g. Mozzi's PDM_VIA_SERIAL

imperfect, but the serial hardware does the timing

• Mozzi's PDM_VIA_I2S

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 this 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 (if your IC happens to have it): Oversampling fast PWM

Basic idea: If we can PWM much faster than the signal's rate, and lowpass the result to the signal's rate.

The lowpassed duty cycle can then be an average stable and close enough to the intended signal. Oversampling PWM makes its output more linear and less noisy (lessening requirement to any filtering), and preferably we want two orders of magnitude faster, order of ~1MHz for a 20kHz signal.

That speed basically requires dedicated PWM circuitry in the IC.

Limitations:

• if you don't have very fast PWM, the net result is high noise

• the lowpass is probably 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 impossible for audio rate, particularly if you also want to do anything else in the microcontroller

• even if the PWM output is regular due to dedicated hardware, we also need to update the sample to put out with strict regularity

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

you'ld really prefer a small buffer, so that you don't need interrupts to be at at audio rate (if it is, expect everything else to be secondary to this). At this point, you're starting to describe a dedicated DAC

• multiple channels may be possible, but how easy this is varies with IC

this might actually help you e.g. produce 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 dedicated PWM pins, at 50MHz(verify), plus a filter and buffer (exact details vary)

Some simple ADCs and DACs for DIY

See Some simple ADCs and DACs

See also

Digital sample capture, storage, reproduction