26
ATmega103(L)
0945G–09/01
Note:
1. The Power-on Reset will not work unless the supply voltage has been below V
POT
(falling).
Power-on Reset
A Power-on Reset (POR) circuit ensures that the device is reset from power-on. As
shown in Figure 23, an internal timer clocked from the Watchdog timer oscillator pre-
vents the MCU from starting until after a certain period after V
CC
has reached the Power-
on Threshold voltage (V
POT
), regardless of the V
CC
rise time (see Figure 24). The Fuse
bits SUT1 and SUT0 are used to select start-up time as indicated in Table 5. A “0” in the
table indicates that the fuse is programmed.
The user can select the start-up time according to typical oscillator start-up time. The
number of WDT oscillator cycles used for each time-out except for SUT = 00 is shown in
Table 6. The frequency of the Watchdog oscillator is voltage-dependent as shown in
“Typical Characteristics” on page 118.
The setting SUT 1/0 = 00 starts the MCU after 5 CPU clock cycles, and can be used
when an external clock signal is applied to the XTAL1 pin. This setting does not use the
WDT oscillator and enables very fast start-up from the sleep modes Power-down or
Power-save if the clock signal is present during sleep. For details, refer to the program-
ming specification starting on page 99.
If the built-in start-up delay is sufficient, RESET can be connected to V
CC
directly or via
an external pull-up resistor. By holding the pin low for a period after V
CC
has been
applied, the Power-on Reset period can be extended. Refer to Figure 25 for a timing
example of this.
Table 5.
Reset Characteristics (V
CC
= 5.0V)
Symbol
Parameter
Condition
Min
Typ
Max
Units
V
POT
(1)
Power-on Reset Threshold
(rising)
1.0
1.4
1.8
V
Power-on Reset Threshold
(falling)
0.4
0.6
0.8
V
V
RST
RESET Pin Threshold Voltage
V
CC
/2
V
T
TOUT
Reset Delay Time-out Period
SUT = 00
5
CPU cycles
SUT = 01
SUT = 10
SUT = 11
0.4
3.2
12.8
0.5
4.0
16.0
0.6
4.8
19.2
ms
Table 6.
Number of Watchdog Oscillator Cycles
SUT 1/0
Time-out at V
CC
= 5V
Number of WDT Cycles
01
0.5 ms
512
10
4.0 ms
4K
11
16.0 ms
16K