Hello everyone,
I am designing my own PCB and I will be using an ATMega32u4 chip on. This is my first time embedding an MCU directly on my board and I want to make sure I go the bare minimum setup correct. I was hoping someone can take a look at the schematics and let me know if they notice anything missing. All I need is a simple reset push button, an ICSP header and access to the digital pins. The device will communicate to the PC via USB micro connector. Let me know
Thanks
I have checked the schematics and it looks pretty good. Although I would suggest you to add single LED for indication.
Some time back I built a timer board for my UV Exposure box. I decided on the 32U4 since it has built in USB. Before I imbedded the board in the light box I tested some other ideas with it. All worked flawlessly.
I have attached my design so you can compare yours to it - just ignore the added baggage.
Noted differences.
Add 0.1uf decoupling caps close to the chip on all VCC pins (you already have one on AREF). This is, in my opinion, mandatory. I did not add any to the AVCC pins - if I were to do it now, I would.
I have a cap to ground on the reset pin. At the time I researched this and decided to add it. With that it is a good idea to have a fly-back diode in parallel to the pull-up resistor as well.
I have a polyfuse (resettable fuse) in line with VUSB, it saved me at least once (had the wrong 5V regulator soldered in).
It is a good idea, as suggested above, to add an onboard LED to the standard pin 13. Helps to check things are working as expected (...upload Blink for first test etc.).
What would I change if I were to do it now? Apart from the extra decoupling cap I would add a TVS (such as SP0503B - I now have a few in my bin) to the usb lines. In the layout I would also try and get the decoupling caps closer to the MCU. On some of my newer boards I added a ferrite bead in the VCC line close to the MCU.
I added LEDs to RX/TX - like to see some activity. I did, however get the polarity wrong and connected the LEDs to ground. Try as I might, I could not invert the way the processor drives the lines. Ended up hacking the board (the red wire).
Note: Some of the above is probably over kill.
One annoyance I found with the 32U4 is that it hangs around for about 8 seconds, waiting for a possible upload, before running the code. That really bothered me. You turn the light box on and nothing happens. I ended using ISP upload to get rid of the bootstrap for normal use - immediate start-up.
Willem
The schematic looks OK, but there are a fair number of "critical layout" issues that it's not clear are understood (I don't like this general style of schematic, where each component gets thrown on the sheet separately, and "connected" just with arrows.) There are bypass caps that should be near the chip. There is a crystal (likewise), and USB traces that might need careful placement...
I appreciate your problem with the schematic format. Initially I really disliked the apparent almost disconnected nature of published Eagle schematics.
I do not know what you use for schematics and board layout, but when I started playing with Arduinos I used Fritzing. However, I found creating parts almost impossible and could not find the footprints I needed. Also laying out boards was a frustrating experience.
I decided to try Eagle and after the initial steep learning curve found the free version could do everything I need - including designing schematics and footprints for all the chips as I need them (if not already available). I now actually appreciate the separated approach. I can design section and connect them by just specifying where they should connect.
When moving to layout, all parts are correctly connected and presented with the necessary "airwires". It is entirely up to you how you handle the placement. Following the rules for critical parts, and traces, is entirely up to the individual. I am a beginner at this and I am learning all the time from other - a big thanks to everyone here as well. The limitations are definitely not the software - it is entirely mine.
I think I am getting better and all my boards are working as planned (that does not necessarily imply correct - layout wise).
I really appreciate critique on my layouts - that is how we learn. Bypass caps as close to the chip is a given - maybe not quite close enough initially. I also try to follow ATMEL's suggested rules for crystal placement, symmetry and ground planes etc.
