AT90USB646 datasheet(361/459 Pages) ATMEL

361 / 459 page

361

7593H–AVR–11/08

ATmega32U6/AT90USB64/128

ATmega32U6/AT90USB64/128 includes a unique 10 bytes serial number located in the signa-

ture row. This unique serial number can be used as a USB serial number in the device

enumeration process. The pointer addresses to access this unique serial number are given in

Table 28-6 on page 361..

Note:

All other addresses are reserved for future use.

28.7.11

Preventing Flash Corruption

During periods of low V

CC, the Flash program can be corrupted because the supply voltage is

too low for the CPU and the Flash to operate properly. These issues are the same as for board

level systems using the Flash, and the same design solutions should be applied.

A Flash program corruption can be caused by two situations when the voltage is too low. First, a

regular write sequence to the Flash requires a minimum voltage to operate correctly. Secondly,

the CPU itself can execute instructions incorrectly, if the supply voltage for executing instructions

is too low.

Flash corruption can easily be avoided by following these design recommendations (one is

sufficient):

If there is no need for a Boot Loader update in the system, program the Boot Loader

Lock bits to prevent any Boot Loader software updates.

Keep the AVR RESET active (low) during periods of insufficient power supply voltage.

This can be done by enabling the internal Brown-out Detector (BOD) if the operating

voltage matches the detection level. If not, an external low V

CC reset protection circuit

can be used. If a reset occurs while a write operation is in progress, the write operation

will be completed provided that the power supply voltage is sufficient.

Keep the AVR core in Power-down sleep mode during periods of low V

CC. This will pre-

vent the CPU from attempting to decode and execute instructions, effectively protecting

the SPMCSR Register and thus the Flash from unintentional writes.

28.7.12

Programming Time for Flash when Using SPM

The calibrated RC Oscillator is used to time Flash accesses. Table 28-7 shows the typical pro-

gramming time for Flash accesses from the CPU.

Table 28-6.

Signature Row Addressing

Signature Byte

Z-Pointer Address

Device Signature Byte 1

0x0000

Device Signature Byte 2

0x0002

Device Signature Byte 3

0x0004

RC Oscillator Calibration Byte

0x0001

Unique Serial Number

From 0x000E to 0x0018

Table 28-7.

SPM Programming Time

Symbol

Min Programming Time

Max Programming Time

Flash write (Page Erase, Page Write,

and write Lock bits by SPM)

3.7 ms

4.5 ms

AT90USB646 Datasheet(PDF) 361 Page - ATMEL Corporation

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