www.austriamicrosystems.com

Revision 1.02

7 - 10

AS1907/AS1908/AS1909

Data Sheet - A p p l i c a t i o n I n f o r m a t i o n

8 Application Information

Duration vs. Reset Comparator Overdrive graph (see page 5) shows the maximum pulse width that a negative-going

mum allowable pulse width decreases.

Valid Reset Output

capabilities greatly decrease. High-impedance CMOS-logic inputs connected to RESET can drift to indeterminate volt-

For the AS1907, where RESET must be valid down to 0V, add a pull-down resistor between RESET and GND (see

Figure 1 on page 1) to eliminate stray leakage currents and hold RESET low. A pull-down resistor of 100k

Ω is large

enough not to load RESET yet small enough to pull it low.

Since the AS1909 has an open-drain, active-low output, it typically uses a pull-up resistor. With this device, RESET will

most likely not maintain an active condition, but will drift to a non-active level due to the pull-up resistor and the

reduced sinking capability of the open drain device. Therefore, this device is not recommended for applications where

Bi-Directional Microprocessor Interface

The RESET output on the AS1909 is open-drain, thus this device can interface easily with microprocessors that have

bi-directional reset pins, such as the Motorola 68HC11.

Connect the RESET output of this device directly to the microprocessor’s RESET I/O pin with a single pull-up resistor

to allow either device to assert reset (see Figure 1 on page 1).

Layout Considerations

The devices require proper layout and design procedures for optimum performance.

!

Short, wide traces should be used to reduce stray inductance and capacitance.

!

Bypass capacitors should be as close to the device as possible.

!

Large ground planes should be used wherever possible.

Table 5. Reset Thresholds

Reset Thresholds

Suffix

Min

Typ

Max

Min

Max

25

2.478

2.50

2.523

2.43

2.58

24

2.378

2.40

2.422

2.33

2.47

23

2.279

2.30

2.321

2.23

2.37

22

2.180

2.20

2.220

2.13

2.27

21

2.081

2.10

2.119

2.04

2.18

20

1.982

2.00

2.018

1.94

2.06

19

1.883

1.90

1.917

1.84

1.96

18

1.784

1.80

1.816

1.75

1.85

17

1.685

1.70

1.715

1.65

1.75

16

1.586

1.60

1.614

1.55

1.65