Rainbow Electronics ATtiny15L - Manual (Page 18)

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ATtiny15L
1187E–AVR–06/02
MCU Status Register –
MCUSR The MCU Status Register provides information on which reset source caused an MCU
Reset.
• Bit 7..4 – Res: Reserved Bits
These bits are reserved bits in the ATtiny15L and always read as zero.
• Bit 3 – WDRF: Watchdog Reset Flag
This bit is set (one) if a Watchdog Reset occurs. The bit is reset (zero) by a Power-on
Reset, or by writing a logical “0” to the flag.
• Bit 2 – BORF: Brown-out Reset Flag
This bit is set (one) if a Brown-out Reset occurs. The bit is reset (zero) by a Power-on
Reset, or by writing a logical “0” to the flag.
• Bit 1 – EXTRF: External Reset Flag
This bit is set (one) if a External Reset occurs. The bit is reset (zero) by a Power-on
Reset, or by writing a logical “0” to the flag.
• Bit 0 – PORF: Power-on Reset Flag
This bit is set (one) if a Power-on Reset occurs. The bit is reset (zero) by writing a logical
“0” 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.
Internal Voltage
Reference ATtiny15L features an internal bandgap reference with a nominal voltage of 1.22V. This
reference is used for Brown-out Detection, and it can be used as an input to the Analog
Comparator. The 2.56V reference to the ADC is generated from the internal bandgap
reference.
Voltage Reference Enable
Signals and Start-up Time The voltage reference has a start-up time that may influence the way it should be used.
The maximum start-up time is 10 µs. To save power, the reference is not always turned
on. The reference is on during the following situations:
1. When the BOD is enabled (by programming the BODEN Fuse).
2. When the bandgap reference is connected to the Analog Comparator (by setting
the AINBG bit in ACSR).
3. When the ADC is enabled.
Thus, when the BOD is not enabled, after setting the AINBG bit, the user must always
allow the reference to start-up before the output from the Analog Comparator is used.
The bandgap reference uses typically 10 µA, and to reduce power consumption in
Power-down mode, the user can avoid the three conditions above to ensure that the ref-
erence is turned off before entering Power-down mode.
Bit 7 6 5 4 3 2 1 0
$34 – – – – WDRF BORF EXTRF PORF MCUSR
Read/Write R R R R R/W R/W R/W R/W
Initial Value 0 0 0 0 See Bit Description