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I can't find a complete answer for this anywhere, and I want to fill the gap in my knowledge of MCUs.

  • In MCUs why do I need any other component except, maybe, a voltage regulator and an external clock?

  • Why do PCBs with embedded MCUs look so complicated?

  • If I were to design a minimal PCB with embedded MCU (say atmega324p), what is the minimum list of components I should consider, and why?

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    \$\begingroup\$ cant find it anywhere? did you start at the source, the vendors website? \$\endgroup\$ – old_timer Apr 18 '17 at 13:49
  • \$\begingroup\$ the minimum for a good percentage is no other external components, many have internal RC (not accurate or at least jitter) oscillators and you can get by without bulk capacitance, etc to get your feet wet, use a ftdi usb to uart board or an stlink header from a nucleo board, etc to both power and get into the chip. for real products you need the additional components for various analog reasons, and those boards often do contain a voltage regulator or some power conversion from say 5V usb to 3.3v for the part. \$\endgroup\$ – old_timer Apr 18 '17 at 13:51
  • \$\begingroup\$ A percentage of the why the components is also in the datasheets from the vendors they describe electrically what you should do with reference designs or other examples. Some of it is just historical experience, RC or other filters to protect the device from whatever power source the user hooks up to it, esd protection, etc. \$\endgroup\$ – old_timer Apr 18 '17 at 13:53
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    \$\begingroup\$ @old_timer I have seen datasheets for eg the atmega324p, it doesn't give the whys, just the hows. \$\endgroup\$ – arynaq Apr 18 '17 at 13:56
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    \$\begingroup\$ some of the whys come from college courses and basic electronics. historical experience. again filtering, bulk capacitance so you dont starve the supply and cause a dip, ground bounce, esd protection, pull ups and pull downs, reset circuits, stuff that is generic. I have used a number of vendors parts without any of this, just the bare part on a breakout board, perhaps sometimes having to jumper the reset to vcc or a few other things clearly tied high or low. In a controlled environment, for a real application you want to have some real protection. \$\endgroup\$ – old_timer Apr 18 '17 at 14:02
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In MCUs why do I need any other component except maybe a voltage regulator and an external clock?

Some of the extra components are actually recommended, even for a minimum complexity design:

  • Capacitors required by the voltage regulator for stability and load/line regulation performance.
  • Protection devices like rectifier diodes, TVS diodes, etc.

OTOH, some MCUs will work without an external clock and/or voltage regulator at all, under certain circumstances (low-power, low-speed, battery-powered applications, for example).

Why do PCBs with embedded MCUs look so complicated?

Because they usually want to provide a minimum (or not so minimum) of functionality for the general purpose applications they're targeted to. As an example:

  • Signal LED's indicating whether the MCU is ON/OFF and/or any serial TX/RX communication is ongoing.
  • Interfacing circuits so that new code can be uploaded to the MCU. This may include USB serial to TTL.
  • Connectivity as required (pin headers, USB, power barrels).
  • Several voltage regulators to provide dual 5V & 3.3V supply rails.
  • Power source selection circuits, to automatically select between DC power from barrel or from USB.

Of course, for a specific application, you could get rid of most of them.

If I were to design a minimal PCB with embedded MCU (say atmega324p) what is the minimum list of components I should consider, and why?

It depends entirely on your application. Under certain circumstances and design goals, the list can go down to just the MCU - nothing else required.

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Some microcontroller indeed only require a bypass cap to operate. Others require some kind of external clock, like a crystal, resonator, R-C, or a digital signal fed in from elsewhere. Newer micros tend to have internal clocks, but these are usually only good to a few percent accuracy.

Boards with micros can be complicated because the system needs to do additional things external to the micro. For example, analog inputs may need to be scaled and conditioned before being presented to a A/D input. Micros are usually used to control stuff. That stuff can be complicated.

As for minimal parts list, see the datasheet. Most micros need ground, power (with bypass cap, of course), maybe external crystal or clock, maybe a filter cap for internal voltage regulator, possibly something to drive the reset input, usually some connections to get the program in, etc. For this information you have to read the datasheet, then also look at your overall system requirements. Those dictate how you'll use the micro, and therefore what else you need to connect to it.

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    \$\begingroup\$ I'd say if these are no outputs even the bypass cap may be superfluous. \$\endgroup\$ – Trevor_G Apr 18 '17 at 13:56
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    \$\begingroup\$ @Trevor: Yes. I guess the Signetics write-only memory didn't need a bypass cap either. \$\endgroup\$ – Olin Lathrop Apr 18 '17 at 14:11
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    \$\begingroup\$ A micro with no outputs you probably don't need the power and ground nets either. \$\endgroup\$ – Colin Apr 18 '17 at 14:16
  • \$\begingroup\$ @Trevor don't forget about internal signals. A microcontroller will almost certainly require bypassing even when it does not change is outputs frequently due to all of the internal activity that results from executing code. \$\endgroup\$ – alex.forencich Apr 18 '17 at 20:26
  • \$\begingroup\$ @alex.forencich yes I know. Running on the internal oscillator though it's with internal signals it's probably not a big issue, trace capacitance may be enough. \$\endgroup\$ – Trevor_G Apr 18 '17 at 21:16
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I assume you're talking about prototype boards here. Those are made complex to make prototyping and debugging easier: they tend to include as many features as possible, limited by the fact that they have to remain generic enough, and by the price tag. Typically, there will be at least some LEDs and buttons, proper clock and reset circuits, proper voltage references, plus the hardware required for communication protocols and debug interfaces that the MCU supports

Then it comes to real projects, many basic MCU designs are much simpler than the corresponding proto boards. There will be no LEDs or buttons, no hardware for unused features, etc. As an example, consider this RFID tag made from a single MCU chip and a coil. This is a bit extreme, but many commercial MCU applications have BoM lists which are finger-countable.

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With some circuits even the external clock is not required.

In fact I have seen circuits with zero external components other than an LED for it to actually do something.

The complexity of the circuitry round it is a function of what you want the application to do more than the need of the micro. Adding resets and crystals and whatever else is required to perform a particular application, increases the part count.

But most modern micros will be quite happy to start on their lonesome.

However, one could argue a micro with zero external components doesn't actually do anything. "If a tree falls in a forrest and there is nobody there to hear it, does it make any noise?" So a power supply would be optional too.

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    \$\begingroup\$ Well, that and the fact that the AVR Arduino ecosystem has come to expect 16MHz while, as I remember, the internal R-C oscillators on the ATMeta series of chips operate at 8MHz. \$\endgroup\$ – ssokolow Apr 18 '17 at 17:42
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  • MCU doesn't need anything more that a supply source to works. Unlike microprocessor, microcontroller have intern memory, build-in clock source (even if not accurate)
  • Because MCU has lots of pins?
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Why do PCBs with embedded MCUs look so complicated?

Depending on the application, there can be the need for:

  • Additional memory (SDRAM, EEPROM, flash)
  • Additional I/O devices such as temperature and humidity sensors, accelerometers, USB interface, WiFi module.
  • Additional I/O connectivity (Analog signal conditioning, GPIO level shifting, buffering and isoation.)
  • Power on reset circuitry.
  • Battery management
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