We developed a board based on AT91SAM7S256. The board works fine
and we are close to production. We planned to use SAM-BA boot assistant integrated into device for in-field firmware upgrade. On our first pre-production lot we noticed that some of the board doesn't enumerate immediately after SAM-BA boot procedure, but it takes a few minutes before the PC recognizes the board (atm6124.sys ATMEL AT91xxxx Test Board in Windows Device Manager).

The SAM-BA procedure we use should be the one indicated by Atmel:
1. put TST pin to '1' (ensuring PA0/1/2 are '1' as well).
2. Power-up board and wait a few seconds
3. back to TST = '0' and power cycle.

We have replaced quartz crystal (18.432 MHz) of one good board with bad one, but no improvements. We checked component values and connection for both crystal capacitors and PLL filter, but they are equal on both boards.

What could be the cause of this unexpected enumeration delay on USB after SAM-BA boot?

Thanks,
Luca