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Topic: Arduino on a breadboard - power from FT232RL and independent 5v supply (Read 369 times) previous topic - next topic

sbaratheon

Hi

I have an arduino on a breadboard, this uses the FT232RL to (a) power the atmega and (b) upload the sketch. This works fine.

I've now put a independent 5v regulator on the breadboard (L7805 with 2 caps) which provides a 5v supply. I've added this as ultimately I'll need to power something which requires too much current FT232RL to provide.

The only changes I've made is that the atmega vcc and gnd goes directly to the independent 5v supply, not the 5v and gnd from the FT232RL. The FT232 is now just connected to the atmega by the tx,rx and DTR. However, when connected like this the atmega will no longer accept new sketches - it seems that it is not connecting to the FT232RL (the atmega does run the existing sketch so its obviously getting the correct voltage).

Please could someone clarify why this is no longer working? Should I connect the atmega with 5v from both the RT232L and the independent 5v supply? Is there any other components which I need for this to work? I couldn't find many resources online on this issue.

I've attached a photo of the breadboard as it's currently wired if this helps.




Thanks


PaulRB

Also I don't see any bypass caps on the atmega. And no series resistor for that led.

sbaratheon

Connect the grounds!
Sorry, which ground do you mean? The yellow jumper connects an atmega gnd to the ground rail.

The LED has a built-in resistor. Where should I put bypass caps?

Thanks

PaulRB

Sorry, which ground do you mean?
The ground for the FT323RL and the ATmega ground. The ATmega cannot communicate with the ft232 without a common ground. This is true of all circuits, except where an opto-isolator is used (then the communication is done with light).

PaulRB

The LED has a built-in resistor.
Really? Really really?
Where should I put bypass caps?
The ATmega has two Vcc pins and two ground pins. All 4 should be connected. For each Vcc pin there should be a 0.1uF ceramic bypass cap placed as close as possible to the Vcc pin, between the pin and ground.

sbaratheon

The ground for the FT323RL and the ATmega ground. The ATmega cannot communicate with the ft232 without a common ground. This is true of all circuits, except where an opto-isolator is used (then the communication is done with light).
Thanks very much - it works now. I really need to get up to scratch on my electronics!

GolamMostafa

Really? Really really?The ATmega has two Vcc pins and two ground pins. All 4 should be connected. For each Vcc pin there should be a 0.1uF ceramic bypass cap placed as close as possible to the Vcc pin, between the pin and ground.


There are no two Vcc pins in the ATmega328P MCU.

Pin-7 (Vcc) is the 5V supply pin for the Digital Part of the ATmega328P. To filter out switching noise due to internal digital activities, it is recommended to connect 0.1uF capacitor very near to Pin-7.

Pin-20 (AVcc) is the 5V supply for the Analog Part (ADC) of the At
Tmega328P. Pin-7 and Pin-20 are electrically isolated; but, in the UNO board they have been made short. Pin-8 (GND) and Pin-22 (GND) are shorted together at the chip level.

Technically, do we need to put 0.1uF capa at Pin-20?

PaulRB

Technically, do we need to put 0.1uF capa at Pin-20?
It's a very good question, @GolamMostafa. I always do that, but I don't know for 100% sure that it is required. I guess you could try just one bypass cap, but if there were any problems, the first thing to try would be a 2nd cap.

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