How should an 8-bit AVR read a counter updated by an interrupt?

Volatile controls relevant compiler access behavior; it does not make a multi-byte access indivisible. If the interrupt updates the counter between the main loop's byte reads, the result can combine bytes from different values. The failure may be rare and concentrated around a low-byte rollover.

Copy the shared value inside a short critical section using the toolchain's supported mechanism, preserving the previous interrupt-enable state. Perform calculations and formatting on the local copy after leaving the critical section. Do not hold interrupts disabled while printing or waiting for a peripheral.

Inspect the generated code and the exact variable width when diagnosing the problem. Also consider writes and read-modify-write operations, which need their own synchronization policy. Test rollover boundaries deliberately and account for the critical section in the timing budget of other interrupt-driven tasks.