Atmega2560 on custom pcb

[Chazza]

Hi
I am designing a custom pcb which at its heart, will have an ATMEGA2560 (ATMEGA2560-16AU) microcontroller. I am trying to work out the circuit I will need to support this chip which I will be programming with the Arduino IDE.
I need to ensure that I have the external oscillator (XTAL) , USB and ICSP header correctly setup so I can add the bootloader and then do updates through the USB port.

Would you kindly look at the circuit I have designed to offer any advice as to any changes required.

Two questions that spring to mind;
Do I need the 1M resistor (R37) under XTAL ?
Should both crystals be the same frequency? The one for the MEGA2560 is 16Mhz and the one for CH340 chip is 12Mhz

Any help on circuit is greatly appreciated.
Thank you
Chazza

[sterretje]

@Chazza

Do I need the 1M resistor (R37) under XTAL ?

I don't know.

@Chazza

Should both crystals be the same frequency?

No. What makes you think that? You should study the datasheet of the CH340 to see which clock frequencies it supports and their effect on the working. Same for the ATmega2560.

[6v6gt]

If you are adding a bootloader to the ATmega2560 you clearly want a serial program (sketch) update method. In that case, I believe that you should also use the /DTR signal from the CH340 chip to reset the MCU prior to the program update. Look at schematics for other CH340/Arduino boards.

[Chazza]

Thank you. I will look at that.