I'm way out of my Comfort Zone, a.k.a. some noob questions

[Bmeij.1]

Until now I have Made some boards with pic 12F1840​ microcontroler, But Now I want to make Something bigger.

Thus I designed a Board For My Rock pi 4​ to use with open plotter, for my boat.

LInk to files: ​https://we.tl/t-9WSUsnKv8T

I hope this works, i dit it with wetransfer, i dont no a other way to link files to my question ​

I used a stm32F091RCTx​

The rock pi 4 has 3 volt tolerant pins ​ and i Wanted to connect My stm32 via spi, so I First translated the spi bus from 3v to 3.3v. Next i conected 2 usarts to 2 isolated rs485 traceivers end 2 usarts to 2 isolated rs422 trancievers. I also implemented a can bus and i connected a cheap barromatric sensor .

With the rs422 busses i think i can implement nmea0183​ with wich i can connect my instruments and sensors, with the can interface i think i can implement a nmea2000 bus and for the rs485 busses i got loads of home made sensor boards. But these are problems for a later date.

This board is still in the design phase and in no way raddy for production, a few things i stil have to decide on, f.i. to keep the eprom on the rpio port which is standard for a rpi hat, and a few other things

​questions:

• ​ I Connected die VBAT pin To All The Other vdd pins with 3.3v, i do not really now if this is right, I'm not going to use a battery, i think this pin is used for the rtc and I'm not going to use that function should i leave tis pin floating or connect it with gnd or vdd?
• Related to that question , should i use a low frequency crystal oscillator, or is that olso for the rtc.
• The 8Mhz cristal needs a resistor to the osc_out pin , i got loads of crystals but i dont have a data sheet for those, and i dont now wat value to use.
• Have i implemented the swd connector the right way , should i put a resistor in the swclk, swdio and nrst path ​for current limeting, if so wat would be a good value for those.
• Do you ,looking at my circuit, have an​y tips, improvements or do you see anything problems. Please let me now.

Wel that is it for now, looking out to your reply's

Best regards

bert meijer​

[01101011]

Hello!

VBAT is used to power up external LSE oscilator / backup registers / BDCR registers / rtc, so - if you don't use them - leave unconnected.

LSE - can be used to RTC, so - you don't need it.

You don't need to use external 8MHz cristal - you can instead use HSI - internal cristal, but HSE - external is better choice, when you use any communication protocols in your device.

Sorry - I can't download your attached file.

Regards. ;)

[Tesla DeLorean]

>>i don't know another way to link files to my question ​

There's a paper clip icon under the message window, you can upload a .ZIP file

[Bmeij.1]

01101011thank you for your advice, i think I will dump the lse but keep the hse, and disconect the Vbat pin .

Clive1 thank you for the tip i wil upload my zip this time​.

[Piranha]

HSI is neither a crystal nor a crystal oscillator, but a RC oscillator. And for low-speed communication on USART, SPI, I2C typically the HSI is good enough.

[Bmeij.1]

And here also My reviced ​circuit and board files.

I connected the eprom to 3.0v , removed the Vbat conection to 3.3v and the lse cristal​.

And I also Found Out that I could have some EMC problems ​ with the isolated rs485 transceiver. If i do it the way the data sheet describes i probably only can fit one, maybe 2. I wil have to look in to this​