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Topic: FT232RL to Atmega UART - is this correct? (Read 5014 times) previous topic - next topic

jtw11

Nov 09, 2012, 03:32 pm Last Edit: Nov 09, 2012, 03:34 pm by jtw11 Reason: 1
Hi guys,

I'm using an FT232RL to interface to the UART on a 1284P, I'm not using a bootloader so I don't feel I need to use the reset pin from the 232RL to reset the Atmega. The below is my schematic, how I intend to include the 232RL in my design.

1) Looking around, it seems 5V is the correct voltage level for the Atmega UARTS - correct? The datasheet says connect VCCIO to VCC to drive out at 5V TTL.

2) I do not need to use the CTS and RTS pins for handshaking do I? I'm not even sure what handshaking is, the 232RL datasheet just talks about it in it's 232RL to MCU example. Are the RX & TX pins enough by themselves for the chip to talk to the Atmega?


retrolefty


Hi guys,

I'm using an FT232RL to interface to the UART on a 1284P, I'm not using a bootloader so I don't feel I need to use the reset pin from the 232RL to reset the Atmega. The below is my schematic, how I intend to include the 232RL in my design.

1) Looking around, it seems 5V is the correct voltage level for the Atmega UARTS - correct? The datasheet says connect VCCIO to VCC to drive out at 5V TTL.

Correct. If however you ever have one of the arduino type boards that operates with a Vcc of 3.3vdc then you would connect VCCIO to the 3v3out pin on the FTDI.

2) I do not need to use the CTS and RTS pins for handshaking do I? I'm not even sure what handshaking is, the 232RL datasheet just talks about it in it's 232RL to MCU example. Are the RX & TX pins enough by themselves for the chip to talk to the Atmega?

Yes, no handshaking signals need wiring to anything for the FTDI to operate.
Lefty




jtw11

Quote
Correct. If however you ever have one of the arduino type boards that operates with a Vcc of 3.3vdc then you would connect VCCIO to the 3v3out pin on the FTDI.


Thanks for this - however, this is being embedded in a final design PCB, so will not change. I'll keep that in mind for the future however!

CrossRoads

Connect Reset to +5V, don't let it float.
Make sure the 100nFs are close to the pins when you lay it out.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

jtw11

I see, the datasheet shows no pull up on the reset line to 5V, and the pin descriptor says it is not required - leave unconnected or pulled up to 5V. However - I shall trust your experience.

CrossRoads

Never had a problem on my Duemilanove's, and it's connected to +5.
http://arduino.cc/en/uploads/Main/arduino-duemilanove-schematic.pdf
I've connected it to +5 on the boards I've designed & built as well, has alwayws worked well.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

jtw11

No resistor on the pull up? Is the reset line high impedance in itself then?

That cap, C10, combined with the reset pull up circuit, is the decoupling cap for the power pins yes - not actually anything to do with the reset circuit in itself?

CrossRoads

Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

Graynomad

All inputs on the FT232 have internal pullup resistors, including RST so it's OK to leave them unconnected.

That said it just doesn't feel right to not connect the pin to VCC.

______
Rob
Rob Gray aka the GRAYnomad www.robgray.com

jtw11

I see - I guess these are very weak pull ups then however if people recommend connecting externally to Vcc.

CrossRoads

I don't see in the datasheet that it says the Reset is actually pulled up, but it does say in several places that if not used, the pin may be left open or be connected to Vcc. I'm with Graynomad, no floating inputs, especially control signals.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

jtw11

Just a thought, the 5V and GND shown on my schematic is not the 5V supply from the USB port, but rather 5V from the same regulator that powers all over 5V devices on the board.

Not using the 5v supply from the USB cable itself shouldn't be a problem should it?

CrossRoads

No, using non-USB 5V should not be a problem.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

Graynomad

That's OK, as long as the GNDs are common.

_______
Rob
Rob Gray aka the GRAYnomad www.robgray.com

jtw11


That's OK, as long as the GNDs are common.

_______
Rob


As in, even though I am not using the USB 5v, I must connect the USB GND to the boards GND for comms to work?

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