Safely establishing a connection between Pokitto and an Arduino Uno

[adekto]

What I understand is SPI is usually the best option if speed is concerned

[uXe]

would it be possible to use USB-serial at the same time as serial over the TX(1) amd RX(0) pins?

No, the Uno uses those two pins for USB serial. It would be possible on a Leonardo for example though, because it uses a separate serial interface for USB, and the TX(1) amd RX(0) pins are referenced as Serial1 instead.

You could always use a ‘SoftwareSerial’ library though, and choose any two pins you like.

As @MLXXXp mentioned, if you go with I2C, connect the grounds of the Uno & Pokitto as well.

[Pharap]

(common resistor value to use is 4.7K, but you can do the math if so inclined).

I’ve decided to attempt I2C next because that diagram looks easiest if it doesn’t have any 3.3V/5V issues.

I’ve just read into Ohms law, so I understand the equation, but I don’t know how to derive the values.

I’ve decided to dig around for some guides and diagrams and I’ve found an arduino one that looks quite promising.

Interestingly it notes:

There has been some confusion about the minimum voltage for a ‘high’ level. For the normal digital input pins of the ATmega328p this is 0.6 * Vcc. However, for the I2C pins it is 0.7 * Vcc

So actually that’s 3.5V, not 3V which possibly complicates matters.

I’ve seen the “Philips Applications Note AN97055” thrown around a lot.
If a BC547 is a sufficient replacement for the mosfets in the diagram then I could certainly attempt that set-up.

You could always use a ‘SoftwareSerial’ library though, and choose any two pins you like.

I may opt for that if I struggle with I2C.
I only have a vague idea of what I’m doing, like a BASIC programmer trying to write C code.

would not break an uno board, just put the chip and crystal on a breadboard

I’m not even sure where the crystal is located, but since I’d need the USB serial, that’s probably not the best option.

What I understand is SPI is usually the best option if speed is concerned

At the moment speed isn’t the issue, I just want a way to be able to transmit data of a reasonable size (e.g. 8 bits in one function call).

I can always move up to SPI later.

[adekto]

At the moment speed isn’t the issue, I just want a way to be able to transmit data of a reasonable size (e.g. 8 bits in one function call).

i think arduino wire is exactly 8 bits via spi, wire is also for i2c
i2c is probebly the easiest one to work with and also easyer to chain with mutliple devices if need be (arduino and esp32 and whatever else you want :P)

[uXe]

Interestingly it notes:

There has been some confusion about the minimum voltage for a ‘high’ level. For the normal digital input pins of the ATmega328p this is 0.6 * Vcc. However, for the I2C pins it is 0.7 * Vcc

So actually that’s 3.5V, not 3V which possibly complicates matters.

…could try anyway and see how reliable or not it is?

I’ve seen the “Philips Applications Note AN97055” thrown around a lot.

Yes, they use that method on the Video Game Shield to safely talk to 3.3V Wii Nunchuks, good explanation at the bottom of this page:

https://www.wayneandlayne.com/projects/video-game-shield/design/

If a BC547 is a sufficient replacement for the mosfets in the diagram then I could certainly attempt that set-up.

NPN Transistor in place of the MOSFETs might not work - someone had an idea here using NPNs though:

http://forum.arduino.cc/index.php?topic=4215.0

…once it starts getting this complicated, the most sane solution might just be a good ol’ level-shifter:

https://www.adafruit.com/category/864

[uXe]

So actually that’s 3.5V, not 3V which possibly complicates matters.

…or just pull-up to 5V instead if it has been established that the Pokitto’s I2C pins are fine with that? Could use one of your diodes for a small voltage drop, to have something-slightly-less-than-5V to pull-up to?

[Pharap]

…once it starts getting this complicated, the most sane solution might just be a good ol’ level-shifter:

I only have two kinds of transistors and I’ve only got one of the other kind,
so at this rate, yeah, it’s looking like I probably am just best off ordering some new parts.

One idea I did have was to somehow abuse an optocoupler to isolate the two halves of the circuit, but I think I’d need more than one and frankly I don’t know if it’s a good idea or not anyway.

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…or just pull-up to 5V instead if it has been established that the Pokitto’s I2C pins are fine with that? Could use one of your diodes for a small voltage drop, to have something-slightly-less-than-5V to pull-up to?

Hrm, I’ll mull it over a bit.

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(Electronics is hard, I think I prefer template metaprogramming :P)

[adekto]

i was wondering, why the uno and not something else Arduino based at 3.3v? ZERO, DUE, Teensy, ESP8266

[Pharap]

Because I already own an Uno and didn’t want to buy any new parts if I could get away with what I’ve already got.
I was hoping it would be a simple matter of a couple of diodes and resistors.