How to reliably power up a microcontroller

Question

I power up my project by plugging a power supply into the power socket.

There's a microcontroller inside. Sometimes it boots successfully, sometimes not. I think it depends on how much I fumble while inserting the power connector. If there's an flaky connection right at the start, perhaps it causes a problem.

First question: is that a likely explanation?

Second question: how can I fix it? Is there some way to deny power until the connection is steady? Or is it better to reset the MCU as described here? (I don't understand how that solution would work though.)

There's a 470uF capacitor on the analog side of the circuit on the 9V side of the 5V regulator which supplies the MCU. Could that be part of the problem?

My main constraints are cost and board space. Power consumption isn't a huge concern: I measured consumption as 70mA, and typically at least 150mA would be available.

Edit

Here are (what I think) are the relevant parts of the circuit. Apologies for not including these earlier.

And here's the analog part of the circuit:

Answer 1

There are "voltage supervisor"- or "reset"-chips for exactly this purpose.

Such chips monitor the supply voltage and enable their output when the supply voltage is time x (usually in the range of 0.5s) over a certain threshold voltage (usually selectable with part number).

The output of those chips is then connected to the reset pin of your MCU in such a way that the MCU is held in reset during time x.

Often times MCUs have similar features integrated. But a dedicated chip is usually the more robust solution since it does not rely on firmware (peripheral configuration).

Another point is robust firmware design. That's a whole different topic, but it's basically about how your firmware reacts to unexpected/unusual events.

For example, if your MCU reads an external EEPROM after bootup, but that EEPROM isn't ready yet. Does your firmware hang itself? Does it continue with corrupt data? Or does it handle this in a more robust way?

Answer 2

80 ohms is too much resistance between the 7805 and the output cap for the regulator to remain stable. The datasheet probably says the max capacitor ESR should be an ohm or less. Move the cap inside the 80ohms, or have a big cap on either side of the 80 ohms.

Also, the USB can backpower the 5v bus too, so plugging one in before the other could conceivably latch up the circuit. Follow the same sequence each time, or protect against the wrong power sequencing.

The part below here is what I wrote first, before I realized you "micro" was a board with reset circuits already. I still feel it's worth saying though:

In general, never ever leave the reset pin floating unconnected.

Anything can couple to its high impedance input, even capacitively, and cause a halt.

Read the processor manual for recommendation on how to tie that pin off properly. And if there are any registers to configure, as well.