The uIoT gateway shield has FTDI plug that I used mostly for serial debugging, while for programming I used a clone of the USBTiny programmer. I wanted to adapt optiboot for the uIoT gateway, as that will make it easier for the end user to upload sketches. The challenges of having a bootloader on it are a few, most important is the changing clock frequency.
The uIoT gateway is clocked from ENC28J60’s clkout pin, that defaults to 6.25Mhz after powering up, and you can set it to 12.5Mhz as I described here. Once you set that higher frequency, it will run @ 12.5Mhz until that is changed by the software or upon power-down/power-up cycle (but not a RESET), so you have two possible situations when trying to upload a sketch – you can be either running @ 6.25Mhz or 12.5Mhz… not cool when it comes to bootloading, since we need to define a baud rate. It also happens that the maximum baud rate that I could get is 38400 @ 6.25Mhz, so if the sketch later sets the frequency to 12.5Mhz the bootloader will actually work @ 38400*2=76800 baud.. yuck, this is non standard baud rate. So I decided to go for 19200 @ 6.25Mhz and if you are running on double speed – that will become the common baud rate of 38400. True it is a bit slower, but it works. It took a while to make optiboot work at 19200 baud rate, mostly it was switching at software UART during compile time so I had to comment out that part of the code. Other adjustments were to blink the led on the uIoT shield properly, as it is on other pin compared to “normal” Arduino. I also created two new entries in the “boards.txt”, adjusted fuses and got it all working 🙂
############################################################## suno.name=Simon K. with Optiboot @6.25Mhz suno.upload.protocol=arduino suno.upload.maximum_size=32256 suno.upload.speed=19200 suno.bootloader.low_fuses=0xFF suno.bootloader.high_fuses=0xDE suno.bootloader.extended_fuses=0x05 suno.bootloader.path=optiboot suno.bootloader.file=optiboot_uiot.hex suno.bootloader.unlock_bits=0x3F suno.bootloader.lock_bits=0x0F suno.build.mcu=atmega328p suno.build.f_cpu=6250000L suno.build.core=arduino suno.build.variant=standard ############################################################## zuno.name=Simon K. with Optiboot @12.50Mhz zuno.upload.protocol=arduino zuno.upload.maximum_size=32256 zuno.upload.speed=38400 zuno.bootloader.low_fuses=0xFF zuno.bootloader.high_fuses=0xDE zuno.bootloader.extended_fuses=0x05 zuno.bootloader.path=optiboot zuno.bootloader.file=optiboot_uiot.hex zuno.bootloader.unlock_bits=0x3F zuno.bootloader.lock_bits=0x0F zuno.build.mcu=atmega328p zuno.build.f_cpu=12500000L zuno.build.core=arduino zuno.build.variant=standard ##############################################################
