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but im assuming it takes signals from usb and then bit bangs it as a serial connection.
No it has a real UART inside it.

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so the usb/fdti chip simply converts usb data signals into TTL signals which are interpreted by the UART, correct? if thats the case could you not just recreate TTL signals through a headphone jack audio signal?
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so the usb/fdti chip simply converts usb data signals into TTL signals which are interpreted by the UART, correct? if thats the case could you not just recreate TTL signals through a headphone jack audio signal?

Audio Signals are AC-coupled, they are not very well suited for NRZ-Codes used by RS232 ( NRZ: http://en.wikipedia.org/wiki/Non-return-to-zero )
This is the reason why I used Manchester Coding ( also use by Ethernet http://en.wikipedia.org/wiki/Manchester_coding ). After some time it turned out that not every PC audio output has the same polarity, therefore I had to switch to differential manchester coding.

The circuit to interface the MC to the PC is in my opinion as minimalistic as possible:



Not all MCs have the same voltage switching level from 0 to 1. They seem to be for the Atmega168 quite similar but may differ to the Atemga328. So the voltage devider should be addapted.
« Last Edit: January 01, 2013, 04:22:43 pm by ChrisMicro » Logged

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Chris, does your bootloader even use the UART? i thought it could technically run on any pin and all the demodulation was taking place in software?
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No, it does not use the UART because the UART can't do Manchester Encoding.
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The audio bootloader can no doubt be made to work (although you would need a way of confirming the process worked, eg. flash an LED) however I can't help you right now because I don't have the hardware to reproduce it.
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so I am running my project off of a rechargeable 3.3v coin cell but the project will be charged using a USB connection. the micro usb has 5 pins so I am considering using power, ground, rx, tx, reset since this will allow for charging and also allow the chip to be reprogrammed. I would like to use an UNO's ftdi chip to program my project but i hear that the UNO is a 5v programmer and it could damage my circuit. How would I allow for 5v ftdi programming? The atmega itself should be fine with 5 volts but my battery and my nrf24l01+ wireless chip probably should not go up to 5volts.

any suggestions?
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You could make your own programming device (based on the Atmega328P) and run it at 3.3V.
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