Hello,
I designed a custom PCB inspired by the Arduino Mega 2560 Rev3. I am facing an issue where the ATmega2560 does not appear to execute any code, and USB uploads via bootloader always fail.
Full EasyEDA schematic is attached.
I will describe the situation chronologically and list all tests performed.
Hardware
Custom PCB based on:
- ATmega2560-16AU
- ATmega16U2-MU (USB–Serial interface)
- 16 MHz crystal on both MCUs
- Design inspired by Arduino Mega 2560 Rev3
USB-C connector with ESD protection.
5V copper plane used for VCC.
ATmega2560 clock configuration:
- 16 MHz crystal
- 1M resistor in parallel
- No external load capacitors on XTAL1 / XTAL2
USB Section (ATmega16U2)
Initially, when plugging the board into USB, nothing appeared in Device Manager.
After investigation, I found that the USB D+ series resistor (R_USB_DP1) was populated with 1 kΩ instead of approximately 22 Ω.
After removing the 1 kΩ resistor and bridging the pads:
- The ATmega16U2 appeared in DFU mode
- I flashed the official firmware:
Arduino-usbserial-atmega16u2-Mega2560-Rev3.hex - The board is now correctly detected as Arduino Mega 2560
- A stable COM port appears in Windows
Conclusion:
The ATmega16U2 and USB interface now appear to be functioning correctly.
ATmega2560 – ISP Programming
Using USBasp:
Initially:
- “Target does not answer”
Programming only works when the “Slow SCK” jumper is enabled.
With Slow SCK enabled:
- Burn Bootloader succeeds
- Upload Using Programmer succeeds
However:
There is no visible evidence that the flashed program is actually executed.
Important detail:
ISP does not work at normal speed, only with Slow SCK enabled.
USB Upload
Using Arduino IDE:
Board: Arduino Mega or Mega 2560
Processor: ATmega2560
Upload via USB consistently fails with errors such as:
- stk500v2_getsync() failed
- timeout communicating with programmer
- avrdude timed out
- sometimes “port disappeared”
Pressing RESET does not cause visible USB re-enumeration.
Conclusion:
Bootloader upload via USB does not work.
Code Execution Test
To verify whether the ATmega2560 is running code, I flashed this minimal sketch using USBasp:
void setup() {
pinMode(44, OUTPUT);
}
void loop() {
digitalWrite(44, HIGH);
delay(1000);
digitalWrite(44, LOW);
delay(1000);
}
Result:
- No observable output activity
- No visible change on pins 44–47
- No indication that the MCU is executing the program
However:
The MOSFET/backlight output stage works when the gate is manually forced to +5V, confirming the output hardware itself is functional.
This suggests the ATmega2560 can be programmed in ISP (Slow SCK), but does not appear to execute code.
Observations
- ISP only works with Slow SCK
- USB bootloader upload always fails
- No user code appears to execute
- No crystal load capacitors are present on the ATmega2560
What would you check next in this situation?
Thank you.
