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As a hobbyist I code in a variety of arduino's and in Digispark's (Attiny 85 and 167).

But I'm doing a project with space contraints, so I'd need to use just the MCU directly over my pcb.

Apart from a voltage regulator (to get the VIn to 5V or 3.3V) do I need extra components to mount the MCU to my PCB?

DO I need crystals, capacitors, etc? Or the MCUs (mostly AtMega 32u4 and Attiny85 and Attiny167) have it all what's needed inside them?

I'll program the MCU before soldering on to the PCB, so there's no need for USB or programming stuff or leds.

Thanks!

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  • Why do you need the voltage regulator? All the MCUs you listed run at 5V, as far as I know. I'd suggest going to SMD components, as those are quite a bit smaller. Hand soldering them isn't that much harder than through-hole.
    – Gerben
    Aug 11, 2020 at 12:18
  • @Gerben I'll use a 7.4V battery as a power supply! Smd is the way I wan to go indeed
    – Rodrigo
    Aug 11, 2020 at 12:55
  • See the datasheet for the regulator you are going to use, to see what capacitors (values and types) it recommends. Then a 0.1µF capacitor right next to each Vcc pin on the MCU. I'd recommend at least putting test pads on the PCB for the programming pins, so when you do have to reprogram them, you can solder some wires to the test pads. Or get a SOIC Clip, if you are using the 8 pin ATTiny MCUs (but make sure there is enough clearance on the PCB for the clip to fit).
    – Gerben
    Aug 11, 2020 at 13:21
  • @Gerben. I´m planning on using an Attiny85. I liked your idea about the clip as a programmer in a soldered SOP8 Attiny. So, apart from having space around for the clip to be able to "attach" the clip to reprogram it, what else should I need besides the bare chip soldered to the PCB? Is that pull-up to the reset pin needed also? Thanks!
    – Rodrigo
    Aug 15, 2020 at 23:35
  • The external pull-up resistor on the reset pin isn't needed per see, but could cause issues in electrically noise environments. Some of my early boards didn't have one, but more recently I've been adding to my PCB. A single 0805 resistor doesn't take that much space. I'd advice to just add it, though I can really find any good rational for this, other than "everybody is doing it".
    – Gerben
    Aug 16, 2020 at 15:22

1 Answer 1

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DO I need crystals, capacitors, etc? Or the MCUs (mostly AtMega 32u4 and Attiny85 and Attiny167) have it all what's needed inside them?

You don't need a crystal if you use the internal clock of the MCU (which means setting the fuses right and compiling your code to expect the right clock frequency). However if you are using a direct USB interface (such as for the ATMega32U4) then you will need a crystal as the precise timing is required for USB to operate.

You will need capacitors. Decoupling capacitors on all the power pins are not optional and must be included.

You will need a pullup resistor on the RESET pin unless you have set the fuse to disable the RESET pin (not advisable since it will prevent ICSP programming in the future - but can be useful for low pin-count chips).

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  • So, If I'm never going back to changing code after soldering, all I need is a decoupling capacitor on the 5V PIN of the MCU? Are there any other places I should use the capacitors? (I'll use a 7.4V Li battery as a power source, and a voltage regulator (which uses already two capacitors on Vin and Vout.)
    – Rodrigo
    Aug 11, 2020 at 13:03
  • You should be fine with that. You'd normally have a 10uF capacitor for a group of chips or where the power enters the board, but the regulator's capacitors will take care of that.
    – Majenko ♦
    Aug 11, 2020 at 13:05
  • 1
    If you're doing ADC reading a capacitor on the AREF pin can help increase reading stability, but it's not required.
    – Majenko ♦
    Aug 11, 2020 at 13:05

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