Datasheet

347
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
Note: All other addresses are reserved for future use.
27.7.11 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):
1. If there is no need for a Boot Loader update in the system, program the Boot Loader Lock bits to pre-
vent any Boot Loader software updates.
2. 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.
3. Keep the AVR core in Power-down sleep mode during periods of low V
CC
. This will prevent the CPU from
attempting to decode and execute instructions, effectively protecting the SPMCSR Register and thus the
Flash from unintentional writes.
27.7.12 Programming Time for Flash when Using SPM
The calibrated RC Oscillator is used to time Flash accesses. This table shows the typical programming time for
Flash accesses from the CPU.
27.7.13 Simple Assembly Code Example for a Boot Loader
;-the routine writes one page of data from RAM to Flash
; the first data location in RAM is pointed to by the Y pointer
; the first data location in Flash is pointed to by the Z-pointer
;-error handling is not included
;-the routine must be placed inside the Boot space
; (at least the Do_spm sub routine). Only code inside NRWW section can
; be read during Self-Programming (Page Erase and Page Write).
;-registers used: r0, r1, temp1 (r16), temp2 (r17), looplo (r24),
Table 27-6. 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
Table 27-7. SPM Programming Time
Symbol Min. Programming Time Max. Programming Time
Flash write (Page Erase, Page Write, and write Lock bits by
SPM)
3.7ms 4.5ms