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ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
28.2.1 Latching of Fuses
The fuse values are latched when the device enters programming mode and changes of the fuse values will
have no effect until the part leaves Programming mode. This does not apply to the EESAVE Fuse which will
take effect once it is programmed. The fuses are also latched on Power-up in Normal mode.
28.3 Signature Bytes
All Atmel microcontrollers have a three-byte signature code which identifies the device. This code can be read in
both serial and parallel mode, also when the device is locked. The three bytes reside in a separate address
space.
ATmega16U4 Signature Bytes:
1. 0x000: 0x1E (indicates manufactured by Atmel).
2. 0x001: 0x94 (indicates 16KB Flash memory).
3. 0x002: 0x88 (indicates ATmega16U4 device).
ATmega32U4 Signature Bytes:
1. 0x000: 0x1E (indicates manufactured by Atmel).
2. 0x001: 0x95 (indicates 32KB Flash memory).
3. 0x002: 0x87 (indicates ATmega32U4 device).
28.4 Calibration Byte
The device has a byte calibration value for the internal RC Oscillator. This byte resides in the high byte of
address 0x000 in the signature address space. During reset, this byte is automatically written into the OSCCAL
Register to ensure correct frequency of the calibrated RC Oscillator.
28.5 Parallel Programming Parameters, Pin Mapping, and Commands
This section describes how to parallel program and verify Flash Program memory, EEPROM Data memory,
Memory Lock bits, and Fuse bits in the device. Pulses are assumed to be at least 250ns unless otherwise
noted.
28.5.1 Signal Names
In this section, some pins of the device are referenced by signal names describing their functionality during
parallel programming, see Figure 28-1 on page 357 and Table 28-6 on page 357. Pins not described in the
following table are referenced by pin names.
The XA1/XA0 pins determine the action executed when the XTAL1 pin is given a positive pulse. The bit coding
is shown in Table 28-9 on page 358.
When pulsing WR
or OE, the command loaded determines the action executed. The different commands are
shown in Table 28-10 on page 358.