Atmel AVR180, Application Note

Page: 7 / 12

AVR180
7
transistor, but take care when selecting a transistor. A
change in this parameter will seriously affect the threshold
voltage of the detector.
R1 and R2
As the formula states, the threshold voltage V
T
is depen-
dent upon R1 and R2. Resistor R1 should be about 200 k
Ω
or lower. This ensures that the current out of the transistor
T1’s base will not influence the voltage divider R1-R2. (This
is for an amplification (
β
/h
FE
) value of at least 100.)
R3
The AVR’s RESET pin has an internal pull-up resistor with
a nominal value of 100-500 k
Ω
. When transistor T1 is off,
the internal pull-up and R3 form a voltage divider. The
resulting RESET voltage has to be sufficiently low to
assure that the MCU RESET line is held active. The recom-
mended value for resistor R3 is 50 k
Ω
or lower, which
ensures that the voltage at RESET is always less than
1/3·V
CC
.
Reset Switch/In-System Programming
If push button reset and/or ISP capabilities are required, a
series resistor R4 must be connected as shown in Figure
3b. This resistor allows the reset switch/programmer to
override the transistor T1 and pull the RESET pin low. To
ensure proper low voltage detector operation, the series
resistance in R3 + R4 should not exceed the recommended
50 k
Ω
.
Threshold Accuracy
As the threshold voltage is defined mainly by R1 and R2,
inaccuracies in these resistors directly influence the thresh-
old voltage. It is recommended to use resistors with ± 1%
tolerance.
Due to the slow transitions on the output of the detector,
variations in RESET threshold in the AVR MCU will lead to
inaccuracies in threshold voltage. This inaccuracy is typi-
cally ± 0.15 volts, which equals ± 3% in a 5V system. (± 5%
at 3.3V). This inaccuracy is lowered by choosing a transis-
tor T1 with higher gain (
β
/h
FE
) which increases the transi-
tion speed.
Power Consumption
The current through the detector in normal operating mode
(sufficiently high V
CC
) is found by:
When switch or programmer force RESET to GND, the cur-
rent increases to:
When voltage drops to the level where the transistor T1
closes, the current drops to:
I
V
CC
R1 R2
+
( )
R3
||
--------------------------------------- V
CC
1
R1 R2
+
---------------------
1
R3
------- +
 
 
=
≈
I
V
CC
R1 R2 +
( )
R3 R4 R
RESET
|| || ||
------------------------------------------------------------------------------
≈
I
V
CC
R1 R2
+
( )
R3 R4 R
RESET
+ +
( )
||
-------------------------------------------------------------------------------------
≈
Table 2. Example Values
Component
Example Values
Recommended Tolerance V
T
= 3.0V V
T
= 4.5V
T1 BC558 / BC858 / 2N3906 I
CE
≥
2.5 mA, V
CE
≥
8 V,
β
/h
FE
≥
100
R1 180 k
Ω ≤
1%
R2 56 k
Ω
33 k
Ω ≤
1%
R3
≤
47 k
Ω ≤
20%
R4
≤
4.7 k
≤
20%