I hope this message finds you well. I've recently developed a minimalist circuit for the Arduino Mega with the intention of uploading the bootloader via SPI pins and utilizing a USB FTDI chip for programming.
Before proceeding further with my project, I would greatly appreciate your assistance in reviewing the attached schematic. My primary concern is to ensure that this minimalist setup aligns with my expectations and can be effectively utilized in my project.
Your expertise and feedback are invaluable to me, and any suggestions or insights you can provide would be immensely helpful in guiding me forward.
C5 is suspect. Recommended for slow vcc rise in the datasheet, but not used on Mega or Uno, and may interfere with auto+reset.
Should have clamping diode on RESET.
USB could use a fuse.
FT232RL is somewhat obsolete at this point…
Regarding the capacitor value for the reset pin, there seems to be a discrepancy between the Arduino Mega reference design and the SparkFun schematic. The Arduino Mega reference design specifies a 22pF capacitor, while SparkFun mentions a 100n_NM capacitor, which I believe refers to a 100nF capacitor. What value should i use here?
i have very little knowlege about electronics. could you give specification about how do i use clamping diodes in reset pin.
In the reference design, i saw they used a 500ma fuse, but i thought is some kind of ferite beeds. i added a 500ma 30v ptc reset fuse to my design.
L1 with 600R DC resistance? Doesn't seem very sensible to me.
I'd forego L1 altogether to be honest. If you want some filtering, the 100nF will handle most of it.
Where's your bulk buffer capacitance? Put 220uF or 470uF on your +5V somewhere.
Where's the fuse?
Where's the protection against a possible external power source trying to feed its 5V back into the USB port? E.g. through H3 or the SPI connector.
If you're going for a minimalist design, why not remove the FTDI from the board and put that onto a separate programmer - or just use a ready-made FTDI programmer? All you need is a header with RX, TX, DTR, GND and +5V or so.
I'd reduce R4 and R5 to 4k7 or even 3k3.
Exactly; I'd also suggest putting a Schottky from RST to Vcc (cathode to Vcc).
I find the net naming around the FTDI very confusing and frankly can't figure out how rx/tx from the Mega end up at the two status LEDs and to the FTDI chip. I suspect you may have duplicate nets that aren't connected, or maybe just very confusing synonyms. You'd have to verify if that part is really the way you intend it to be.
Me too. Drawbacks: you need 3 more components, and the footprint is relatively large compared to the smaller CH340's that don't need a crystal. Total cost will be lower, too. The crystal-less CH340's also work fine.
Note that CH340G is the later version and has been introduced for better sync with 12 MHz USB signal in contrast with the inaccuracies of the previous internal oscillator based CH340.
I'd use whatever CH340 is convenient, or something like a CP2102 if so desired. In the end, I'd go for what's the most easily and cheaply available. No need to make this any more complicated than necessary.
And again, I'd suggest taking the whole UART bridge off of the board to begin with.
I'm sorry, I don't understand what you mean. It's probably due to a language/translation error.
If you mean to say "the design specification demands an UART bridge" - then yes, in a literal sense. However, I think you're also aware that design specifications as we see them here on the forum are generally quite loose/informal, open to interpretation and often change as someone is added alternative options.
That's what may happen here; in this case there's an inherent conflict between "minimal implementation" and the addition of a UART bridge, which is not really required in a minimal implementation as there are plenty of separate USB-UART devices available. Moreover, I think I'm probably not the only one who at some point concluded "gee, I might as well not throw around CH340's/CP2102's all over the place if they end up being used a handful of times anyway". I'm offering that point of view for consideration.
As to the choice of the USB-UART bridge, I feel we've offered sufficient options and can safely conclude that pretty much any approach will work fine.
I made this circuit as a basic setup inorder to work- Like upload a bootloader thorugh spi pins and program through usb. Other power selector circuit will add later. H3 is output, cant be use as input(in my project)
I also think about that, But if i need a usb communication to a PC, i need a onboard usb setup. otherwize there will be loose connections.
Can you explain how you calculated the resistor value?
Here the both uart lines form mega and ftdi are connected
That rx,tx blinking led are connected to the 22 and 23 pins of FTDI
Sorry of the net namings.. i usually add the name of the main microcontroller with the other names. here its "atmega2560_ftditx" is atmega2560 controller - ftdi chip - tx pin.
You're right; the penny didn't drop with me- sorry about that!
No, I mean something that's about 10000 times bigger. So 220-470uF (not nF). You need both the caps you've shown, as well as a bulk capacitor on the board. See the energy supply to the board as a waterpipe. If the pipe gets congested for some reason, or you try to draw a little more from it than its thickness allows, it helps to have a big bucket of water you can take the supply from instead of directly from the pipe. The pipe is the USB cable, the bucket is the bulk capacitance. What you're doing now, is placing a small cup of water next to each component, and that helps, but the cup is emptied quickly in case of a sudden need for more water. Placing a bucket somewhere nearby so the cups can be filled from the bucket helps with that. The analogy isn't perfect, but it's fairly accurate.
It can. Anything connected to that which happens to have its own power supply, will try to supply power to your board and the host USB port if the supply happens to be a little higher.
You can easily connect an FTDI programmer/'cable' to a pin header. I use a default 5-pin single row 2.54mm pitch header for this, or if I want it to be a little more sturdy, a 5-pin JST connector. Works fine.
I didn't - I'm relying on the knowledge that you can easily draw 1mA through the pullups, regardless, and that the lower impedance of 3-5k will help resolve the marginal situation that 10k offers especially if you happen to connect something to I2C with a longish cable (i.e. more than 10cm or so). 10k is really marginal.
No need to be, I was just pointing out that I found it hard to comment on that part of the schematic due to the naming. As long as you are OK with it, so am I!