Rainbow Electronics ATtiny2313 - Manual (Page 34)

34
ATtiny2313
2543A–AVR–08/03
System Control and
Reset
Resetting the AVR During reset, all I/O Registers are set to their initial values, and the program starts exe-
cution from the Reset Vector. The instruction placed at the Reset Vector must be an
RJMP – Relative Jump – instruction to the reset handling routine. If the program never
enables an interrupt source, the Interrupt Vectors are not used, and regular program
code can be placed at these locations. The circuit diagram in Figure 14 shows the reset
logic. Table 15 defines the electrical parameters of the reset circuitry.
The I/O ports of the AVR are immediately reset to their initial state when a reset source
goes active. This does not require any clock source to be running.
After all reset sources have gone inactive, a delay counter is invoked, stretching the
internal reset. This allows the power to reach a stable level before normal operation
starts. The time-out period of the delay counter is defined by the user through the SUT
and CKSEL Fuses. The different selections for the delay period are presented in “Clock
Sources” on page 23.
Reset Sources The ATtiny2313 has four sources of reset:
• Power-on Reset. The MCU is reset when the supply voltage is below the Power-on
Reset threshold (V
POT
).
• External Reset. The MCU is reset when a low level is present on the RESET pin for
longer than the minimum pulse length.
• Watchdog Reset. The MCU is reset when the Watchdog Timer period expires, the
Watchdog is enabled, and Watchdog Interrupt is disabled.
• Brown-out Reset. The MCU is reset when the supply voltage V
CC
is below the
Brown-out Reset threshold (V
BOT
) and the Brown-out Detector is enabled.
Figure 14. Reset Logic
MCU Status
Register (MCUSR)
Brown-out
Reset Circuit BODLEVEL [2..0]
Delay Counters
CKSEL[3:0]
CK
TIMEOUT
WDRF BORF
EXTRF
PORF
DATA BUS
Clock
Generator
SPIKE
FILTER
Pull-up Resistor
Watchdog
Oscillator
SUT[1:0]
Power-on Reset
Circuit