Arduino-compatible minimal board tutorial (provided by Nick Gammon)

Hi there,

For the following questions, i will be referring to the following tutorial posted by Nick Gammon.

  1. Is there any particular reason why the baud rate is set to 115200? Could it be set to 9600 instead?
  2. I plan on making a minimal board running at 8MHz, powered by a 3.7V lithium ion battery (voltage will vary between 3.6-4.2V). If i want to send 3.3V TTL serial between the minimal board and another device (also 3.3V), how can i ensure i have 3.3V if the input voltage of the minimal board can vary? Do i need to do some level shifting?

I would greatly appreciate any help.

Cheers,
Jeff

Nick Gammon uses his bootloader sketch with 115200 baud.
You can change the "const unsigned long BAUD_RATE = 115200;" in that sketch to 9600 and set the serial monitor to 9600.

What is the other device, and what kind of signals do you want to send ?

Thanks for your response.

I plan on getting a 3.3V Bluetooth Serial device.

Should i put a resistor from the ATMEGA328 transmit pin to reduce the voltage going to the serial device to cater for the varying input voltage of the lipo battery of the ATMEGA328? What is the minimum voltage for a logic HIGH?

Thanks for any assistance.
Jeff

Jeffro_Aus:
Should i put a resistor from the ATMEGA328 transmit pin to reduce the voltage going to the serial device to cater for the varying input voltage of the lipo battery

No a single resistor does not reduce voltage.

No a single resistor does not reduce voltage.

Should I use a voltage divider? What voltage range is considered high here?

Cheers

Which 3.3V Bluetooth device ?

This one is a complete shield:

What voltage range is considered high here?

Here and in virtually all electronics if it is higher than the supply voltage it is too high.

Should I use a voltage divider?

Yes.

Which 3.3V Bluetooth device ?

I will get my hands on some HC-05.

Should I use a voltage divider?
Yes.

What value resistors should be considered here because won't the serial transmit voltage from the ATMEGA328 vary with the input from the 3.7V lipo battery (say 3.5-4.2V)? What voltage range would be safe to be considered a HIGH for the 3.3V serial device?

Is there a way to get a fixed 3.3V from the serial transmit pin from the ATMEGA328 regardless of input voltage?

Thanks,
Jeff

Cheap 2-Way Bluetooth Connection Between Arduino and PC : 5 Steps (with Pictures) - Instructables

I came across this also, but i'm not sure how things change when a lipo battery is powering the standalone Atmega328 chip. I can boost the voltage to 5V to power the bluetooth device. But for the serial 3.3V, won't that vary with the lipo input voltage into the Atmega328?

You are right. Sorry, I did forget about that.
The HC-05 bluetooth module doesn't have 5V tolerant I/O pins and it needs 3.3V to power it.
So you have to make a steady 3.3V anyway. Can you use that for the Arduino too ?

If that is a problem, there must be some logic gates that can do this.
The 74HC125 is often used for level shifting, but I don't know if it can be used in this situation.

If that is a problem, there must be some logic gates that can do this.
The 74HC125 is often used for level shifting, but I don't know if it can be used in this situation.

Yeah, i think i am after some logic level converters that can be used to convert to 3.3V from a varying voltage (3.5-4.2V from lipo battery source) on the serial transmit pin. But i am not sure if they exist??

You need a voltage regulator but because the output voltage is so close to the input voltage you need a "buck boost" switching regulator.
Once you have that you can use the circuit in that link.

You need a voltage regulator but because the output voltage is so close to the input voltage you need a "buck boost" switching regulator. Once you have that you can use the circuit in that link.

How do i go about using a "buck boost" switching regulator? Basically i am after a very low power "bare bones" setup here.

Thanks for your help!

Basically i am after a very low power "bare bones" setup here.

Using switching power circuits is not something amenable to a bare bones approach. I would not attempt to build one my self.
This is quite cheap and should do exactly what you want.