I'd seriously lean toward running the programmer at 3.3V if possible, but if not, bi-directional level shifting seems like overkill. I'd breadboard it up just to convince myself, but that's the way I'm reading it. I've done ICSP programming at 3V and 3.3V as well as at 5V, flash as well as EEPROM should work down to 1.8V per the datasheet. I'd either level-shift the programming signals or why not just run the programmer on 3.3V too. So I wouldn't drive an MCU running on 3.3V with 5V signals. The datasheet says that the voltage on any pin (except RESET - for high-voltage programming) cannot be less than -0.5V or more than Vcc + 0.5V. Well I had started to write that all should be fine, then a light went on.
Do the chips come from the factory with the ability to be programmed at 3.3V? Hopefully that clarifies my intentions.ĭo you have any new recommendations based on my interface design? Everything is theoretical at this point, so I can change anything. Does the flash EEPROM memory require 5.0V? I'm also not sure how to handle the interface or (for example) fuse programming. My other idea was to use a 4-bit bidirectional logic level converter (buffer) to convert the SPI lines, but I am still questioning the ability of the 328 to be programmed while operating at 3.3V. I'm just worried about interfacing the SCLK, MISO, MOSI, and RESET pins. That way, the 5.0V would be stepped down to 3.3V before reaching the 328 or the sensors and other components. My plan up until this point was to tie the VCC lines of the ISP programmer to the 5.0V in line on my board. The only lines that will "cross" the 5.0V/3.3V barrier are the ISP/SPI SCLK, MISO, MOSI, and RESET lines. My current circuit layout looks something like this:ĥ.0V in -> 3.3V Regulator -> ATmega328p and Sensors Jack, just for clarification, I plan on bit-banging the serial protocol (SPI) for all of my external devices, so the internal SPI hardware on the ATmega328p will be reserved only for in-system programming. a jumper which when pulled disconnects everything but the MCU from the supply. Something similar could be done on the target board itself, i.e. I use Adafruit's USBtinyISP which has a jumper to optionally prevent the programmer from putting power to the ICSP header (in which case the MCU will need to be powered via its intended 3.3V supply). But like you said, the concern is with whatever other components would be exposed to the 5V from the programmer. There's no worries about the ATmega328 itself being programmed on 5V even if you're expecting to run it at 3.3V.
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