SLVSD43C May   2015  – February 2019

PRODUCTION DATA.

1. Features
2. Applications
3. Description
4. Revision History
5. Pin Configuration and Functions
6. Specifications
7. Detailed Description
1. 7.1 Overview
2. 7.2 Functional Block Diagram
3. 7.3 Feature Description
4. 7.4 Device Functional Modes
8. Application and Implementation
1. 8.1 Application Information
2. 8.2 Typical Application
1. 8.2.1 Design Requirements
2. 8.2.2 Detailed Design Procedure
3. 8.2.3 Application Curves
9. Power Supply Recommendations
10. 10Layout
11. 11Device and Documentation Support
12. 12Mechanical, Packaging, and Orderable Information

• PWP|28
• PWP|28

#### 7.3.7.1 Window Watchdog (WTS, ROSC, FSEL and WRS)

This device works in the window watchdog mode when the watchdog type selection (WTS) pin is connected to a to low voltage level. The user can set the duration of the watchdog window by connecting an external resistor (RROSC) to ground at the ROSC pin and setting the voltage level at the FSEL pin. The current through the RROSC resistor sets the clock frequency of the internal oscillator. The user can adjust the duration of the watchdog window (the watchdog timer period) by changing the resistor value. A high voltage level at the FSEL pin sets the watchdog window duration to 5 times as long as that of a low voltage level with same external component configuration.

The duration of the watchdog window and the duration of the fault output are multiples of the internal oscillator frequency, as shown by the following equations:

 FSEL low Equation 3. t(WD) = RROSC × 0.5 × 10-6 FSEL high Equation 4. t(WD) = RROSC × 2.5 × 10-6 Watchdog initialization Equation 5. t(WD_INI) = 8 × t(WD) Open and closed windows Equation 6. t(WD) = t(OW) + t(CW) WRS low Equation 7. t(OW) = t(CW) = 50% × t(WD) WRS high Equation 8. t(OW) = 8 × t(CW) = (8 / 9) × t(WD)

where:

• t(WD) is the duration of the watchdog window
• RROSC is the resistor connected at the ROSC pin
• t(WD_INI) is the duration of the watchdog initialization
• t(OW) is the duration of the open watchdog window
• t(CW) is the duration of the closed watchdog window

For all the foregoing items, the unit of resistance is Ω and the unit of time is s.

Table 1 illustrates several periods of watchdog window with typical conditions.

### Table 1. Several Typical Periods of Watchdog Window

FSEL R(ROSC) (kΩ) I(ROSC) (µA) t(WD) (ms) WATCHDOG PERIOD TOLERANCE
High 200 5 500 15%
100 10 250 10%
50 20 125
40 25 100
25 40 62.5
20 50 50
Low 100 10 50 10%
50 20 25
40 25 20
25 40 12.5
20 50 10

As shown in Figure 23, each watchdog window consists of an open window and a closed window. While the window ratio selection (WRS) pin is low, each open window (t(OW)) and closed window (t(CW)) having a width approximately 50% of the watchdog window (t(WD)). While the WRS pin is high, the ratio between open window and closed window is about 8:1. However, there is an exception to this; the first open window after watchdog initialization (t(WD_INI)) is eight times the duration of the watchdog window. The watchdog must receive the service signal (by software, external microcontroller, and so forth) during this initialization open window.

A watchdog fault occurs when servicing the watchdog during a closed window, or not servicing during an open window.