Mcu Status Register - Mcusr; Internal Voltage Reference - Atmel ATtiny25 Manual

8.7 MCU Status Register – MCUSR
8.8

Internal Voltage Reference

8.8.1 Voltage Reference Enable Signals and Start-up Time
ATtiny25/45/85
40
The MCU Status Register provides information on which reset source caused an MCU Reset.
Bit 7
–
Read/Write R
Initial Value 0
• Bits 7..4 – Res: Reserved Bits
These bits are reserved bits in the ATtiny25/45/85 and will always read as zero.
• Bit 3 – WDRF: Watchdog Reset Flag
This bit is set if a Watchdog Reset occurs. The bit is reset by a Power-on Reset, or by writing a
logic zero to the flag.
• Bit 2 – BORF: Brown-out Reset Flag
This bit is set if a Brown-out Reset occurs. The bit is reset by a Power-on Reset, or by writing a
logic zero to the flag.
• Bit 1 – EXTRF: External Reset Flag
This bit is set if an External Reset occurs. The bit is reset by a Power-on Reset, or by writing a
logic zero to the flag.
• Bit 0 – PORF: Power-on Reset Flag
This bit is set if a Power-on Reset occurs. The bit is reset only by writing a logic zero to the flag.
To make use of the Reset Flags to identify a reset condition, the user should read and then reset
the MCUSR as early as possible in the program. If the register is cleared before another reset
occurs, the source of the reset can be found by examining the Reset Flags.
ATtiny25/45/85 features an internal bandgap reference. This reference is used for Brown-out
Detection, and it can be used as an input to the Analog Comparator or the ADC.
The voltage reference has a start-up time that may influence the way it should be used. The
start-up time is given in
reference is on during the following situations:
1. When the BOD is enabled (by programming the BODLEVEL [2..0] Fuse bits).
2. When the bandgap reference is connected to the Analog Comparator (by setting the
ACBG bit in ACSR).
3. When the ADC is enabled.
Thus, when the BOD is not enabled, after setting the ACBG bit or enabling the ADC, the user
must always allow the reference to start up before the output from the Analog Comparator or
6 5 4
– – –
R R R
0 0 0
Table 8-4. To save power, the reference is not always turned on. The
3 2 1
WDRF BORF EXTRF
R/W R/W R/W
See Bit Description
0
PORF MCUSR
R/W
7598H–AVR–07/09