Problem downloading EEPROM from Arduino Mega 2560

[ravenboy]

Mac Big Sur, avrdude 6.3

in a sketch running on a Mega2650: using EEPROM library I wrote "757343238393515142B0" starting at location Zero

char buffer[] = "757343238393515142B0";
for (int i=0; i<21; i++) {
EEPROM.write(i,buffer[i]);
}

reading it works fine

char buffer2[21]
for (int i=0; i<21; i++) {
buffer2[i] = EEPROM.read(i);
}

buffer2[20]='\0';
Serial.printlin (buffer2);
prints "757343238393515142B0" to serial

when i download the EEPROM memory using AVRDude, i get no errors.
A printout of the hex file reveals :
b':2000000037353733343332333432423000FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF79\n:20002000FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFE0\n:20004000FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFC0\n

etcetera

which has only 12 of the bytes i wrote to EEPROM
hex 37 35 37 33 34 33 32 33 34 32 42 30
ascii 7 5 7 3 4 3 2 3 4 2 B 0

i wrote 20 bytes 757343238393515142B0
so the download doesn't have 8 bytes in the middle.
the bytes missing start at position 7 and are the 8 bytes 83935151
so the download has the first 7 bytes of what i wrote
plus the last 4, but lacks bytes 8 thru 15

any idea how avrdude's download would disagree with what the arduino sketch can READ??

i used
avrdude -p m2560 -c wiring -P [port] -U eeprom:r:file.hex:i

[mcuee]

That is probably becasue the wiring bootloader has an issue with EEPROM read. You need to build the bootloader again with this patch.

Ref:

• Fix stk500v2 READ_EEPROM to properly read EEPROM arduino/ArduinoCore-avr#24
• avrdude EEPROM read/write problem with optiboot and stk500v2 bootloader #986
• [bug #61169] Bad EEPROM read on mega2560 R3 Arduino board #515

[mcuee]

I will close this issue now. Please re-open if the above fix does not sort out your issue.

[ravenboy]

many thanks!

[ravenboy]

Thanks so much for the speedy and excellent support. Once i sussed out burning bootloaders, burning with stk500boot_v2_mega2560_eeprom_fix.hex is working!!!