Datasheet
348
7766F–AVR–11/10
ATmega16/32U4
Note: 1. The SPIEN Fuse is not accessible in serial programming mode.
2. The default value of BOOTSZ1..0 results in maximum Boot Size. See Table 27-8 on page 344
for details.
3. See “Watchdog Timer” on page 55 for details.
4. Never ship a product with the OCDEN Fuse programmed regardless of the setting of Lock bits
and JTAGEN Fuse. A programmed OCDEN Fuse enables some parts of the clock system to
be running in all sleep modes. This may increase the power consumption.
Note: 1. The default setting of CKSEL3..0 results in Low Power Crystal Oscillator for ATmega16U4 and
ATmega32U4, and Internal RC oscillator for ATmega16U4RC and ATmega32U4RC. See
Table 6-1 on page 28 for details.
2. The CKOUT Fuse allow the system clock to be output on PORTC7. See “Clock Output Buffer”
on page 37 for details.
3. See “System Clock Prescaler” on page 37 for details.
The status of the Fuse bits is not affected by Chip Erase. Note that the Fuse bits are locked if
Lock bit1 (LB1) is programmed. Program the Fuse bits before programming the Lock bits.
Table 28-4. Fuse High Byte
Fuse High Byte Bit No Description Default Value
OCDEN
(4)
7 Enable OCD
1 (unprogrammed, OCD
disabled)
JTAGEN 6 Enable JTAG
0 (programmed, JTAG
enabled)
SPIEN
(1)
5
Enable Serial Program and Data
Downloading
0 (programmed, SPI prog.
enabled)
WDTON
(3)
4 Watchdog Timer always on 1 (unprogrammed)
EESAVE 3
EEPROM memory is preserved
through the Chip Erase
1 (unprogrammed, EEPROM
preserved)
BOOTSZ1 2
Select Boot Size (see Table 28-7
for details)
0 (programmed)
(2)
BOOTSZ0 1
Select Boot Size (see Table 28-7
for details)
0 (programmed)
(2)
BOOTRST 0
Select Bootloader Address as
Reset Vector
1 (unprogrammed, Reset
vector @0x0000)
Table 28-5. Fuse Low Byte
Fuse Low Byte Bit Nr Description Default Value
ATmega16U4/32U4 ATmega16U4RC/32U4RC
CKDIV8
(3)
7 Divide clock by 8 0 (programmed)
CKOUT
(2)
6 Clock output 1 (unprogrammed)
SUT1 5 Select start-up time 0 (programmed)
SUT0 4 Select start-up time 1 (unprogrammed)
CKSEL3 3 Select Clock source 1 (unprogrammed)
(1)
0 (programmed)
(1)
CKSEL2 2 Select Clock source 1 (unprogrammed)
(1)
0 (programmed)
(1)
CKSEL1 1 Select Clock source 1 (unprogrammed)
(1)
1 (unprogrammed)
(1)
CKSEL0 0 Select Clock source 0 (programmed)
(1)
0 (programmed)
(1)