atmegaavrbootloaderusb
I'm designing my first circuit with an ATmega 32u4 processor. Since I read about the BOOTRST and HWBE fuses I'm confused about the programming of the chip.
If
then will it be possible to upload a program to the device with the help of the built in USB boot loader?
There is a fuse on AVRs called RSTDISBL, which disables the functionality of the reset pin and allows you to use it as a normal I/O pin. If you program this fuse then resets will obviously not work and you will not be able to program the chip via ISP. This also means you can't unprogram the fuse via normal methods.
If this is the case then you can unprogram this fuse via High Voltage Programming. If you have an AVR Dragon board handy then you can do this easily by connecting the right pins on the HVSP header to your chip. Be careful if you're doing this in-circuit though, as the high voltage programming options uses 12 volts, which may fry the rest of your circuit. Best is to program it exclusively on the dragon via a ZIF socket or similar.
Here is a video from Atmel on how to unprogram the fuse via High Voltage Parallel Programming: http://www.youtube.com/watch?v=32CkOkpOcSc
I recommend you verify with FTDI datasheet as well as their support pages (they have a decent knowledge base). I have done a similar design:
You can use any of the CBUS pins of the FT232 to connect to the nBootmode pin of your chip. If the signal is the wrong way, then you can program the FT232 EEPROM to invert it. This means as FT232 powers up, it'll set those pins. You might want to employ a similar thing with nReset of your MCU, that way it'll remain in reset mode until FT232 is up and ready.
You'll likely need to manage separating the power bus using diodes such as a BAT54C or schottky diodes likes this:
Think about what would happen if both power ports are [accidentally] plugged in.
Regarding the PWREN#. It is meant for shutting down power of an external chip during USB suspend. It might be tricky in this case since it is low during USB enum, but it only goes high again during USB suspend which might prevent you from interacting with MCU. You might want use a generic CBUS IO and flick it up or down using the driver or one of the UART lines, etc.
Finally, most FT232 chips can't supply more than 50mA of current. Check that your MCU power requirement is lower.
Best Answer
No - take a look at page 3 of this PDF. All entry to the DFU bootloader is done via reset (which you can't access) or a jump from user code (which you don't have on there because it's a blank IC).
However, how are you intending to configure the device in the first place? You'll want to set your fuses correctly, right, which means access to the ICSP pins, which means access to the reset pin...
But in short, without access to the reset pin this is a no-go.