AVR Dragon and 7-seg displays with external power source

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Hello there,

(long introduction, sorry, the question is at the end)

I am playing with the AVR Dragon and seven segments displays. I habe builded all the circuitery for the 7-seg displays, and it is working fine with an external power source.

I am using cathode common displays and NPN transistors to work as switches, that will connect the common pin of the display to the GND of my power source when I put a 5v signal on the base of the transistor. Then I have the terminals of the displays connected to the +Vcc of my power source, and I am abble to light the segments.

OK, so far so good, but I have not used the AVR Dragon yet. My problem is the following:

When using the AVR Dragon with an Atmega32, I will have one pin connected to the base of the transitor. Other pins will be connected to the terminals of the displays. With that, the GND of the external power source will be used on the emiter termal of the transistor (it will reach the common terminal of the display when I put a high signal at the AVR pin connected to it), and other pin of the AVR will have a high signal to light a segment of the display.

Ok, after this long introduction my question is:

What do I do with the +Vcc pin of my external power souce? Can I just let it disconnected? Must I connect it to one of the Vcc pins of the Dragon? Must I use my external power source to power the Atmega?

Dunno if my text is confusing, hope you can understand what my problem is. Please help, I am afraid I can damage something by just trying the three options above :P

Regards,

Uderman

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All you answers are in the Dragon's docs in the Studio's help file.

Quote:
Must I connect it to one of the Vcc pins of the Dragon?
DEFINETELY NOT!! Just the grounds need to be connected.

Are you planning on using JTAG for debugging?

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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js wrote:
All you answers are in the Dragon's docs in the Studio's help file.
Quote:
Must I connect it to one of the Vcc pins of the Dragon?
DEFINETELY NOT!! Just the grounds need to be connected.

Are you planning on using JTAG for debugging?

Hello there js. Thank you for the quick answer.

Yes, I am planning to use JTAG for debugging.

Regards,

Uderman

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Quote:
I am planning to use JTAG for debugging.
In which case you need to connect the target (ie your external board) VCC to the JTAG header so that the Dragon can read the target voltage, this also goes for the ISP header.

However the Dragon's VCC from the GND/VCC header MUST NOT be connected to the target's VCC or the 2 will fight and the Dragon will lose.

If the target's current requirements are low, maybe 50mA max and your USB port can supply the current then you can use the Dragon's VCC to provide the power, but I always try and use another power supply so that I don't strecth the friendship with the Dragon. It gets very touchy.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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Thank you again for your help js. I have checked the Dragon manual, and looks like I have to connect my target Vcc to the PIN 4 (VTref) of the JTAG Header.

Indeed I was needing to check the manual with more atention :)

REgards,

Uderman