Application of "Watchdog" Circuit DS1232 in Single-Chip Microcomputer Products

With the development of computer technology, single-chip microcomputers have been widely used in the field of industrial automation and intelligent products. How to improve the anti-interference ability of single-chip microcomputer products is a problem that product developers and designers face and must solve. The specific methods of anti-interference are discussed in detail in many books and articles, so this article will not repeat them. The "Watchdog (WATCHDOG)" integrated circuit DS1232 produced by DALLAS Company in the United States has the characteristics of reliable performance, simple use and low price. It can be used in single-chip microcomputer products to improve the anti-interference ability of hardware. We have applied DS1232 to "grain center warehouse granary temperature monitoring system", "bank interest rate display screen", "multi-function computer clock", "power plant belt scale speed measurement system" and "full-automatic power limit metering system", and received good results in actual use.

1. Structure and features of DS1232
1.1 Pin functions and internal structure

DS1232 is a microprocessor monitoring circuit produced by DALLAS, USA, in an 8-pin DIP package, as shown in Figure 1.
The functions of each pin are as follows:
PBRST: button reset input;
TD: watchdog timer delay setting terminal;
TOL: 5% or 10% voltage monitoring selection terminal;
GND: power ground;
RST: high level active reset output terminal;
RST: low level active reset output terminal;
ST: cycle input terminal;
Vcc: power supply.

1.2 Internal structure of DS1232

The internal structure block diagram of DS1232 is shown in Figure 2.

1.3 Main Features
DS1232 has the following features:
● Available in 8-pin DIP package and 16-pin SOIC package to meet different design requirements;
● Can stop and restart the microprocessor when it is out of control;
● Automatically reset the microprocessor when the microprocessor loses power or the power supply voltage changes transiently;
● Accurate 5% or 10% power supply monitoring;
● No discrete components required;
● Wide temperature range, -40 to +85°C.

2. DS1232 Functions
2.1 Power Supply Voltage Monitoring

DS1232 can monitor the power supply voltage of the microprocessor in real time. When the power supply voltage VCC is lower than the preset value, the 5th and 6th pins of DS1232 output complementary reset signals RST and RST. The preset value is set by pin 3 (TOL); when TOL is grounded, RST and RST signals are generated when the power supply voltage drops below 4.75V; when TOL is connected to VCC, RST and RST signals are generated only when VCC drops below 4.5V. When the power supply returns to normal, the RST and RST signals are maintained for at least 250ms to ensure the normal reset of the microprocessor.

2.2 Button reset

In single-chip microcomputer products, the simplest button reset circuit is composed of resistors and capacitors. If the system expansion has other interface chips that need to be reset at the same time as the microprocessor, this simple resistor-capacitor reset circuit often cannot meet the overall reset requirements. DS1232 provides an input terminal PBRST (pin 1) that can be directly connected to the reset button. When a low-level signal is input to this pin, a reset signal of at least 250ms will be output at the RST and RST terminals. The specific circuit is shown in Figure 3.

2.3 Watchdog Timer

The DS1232 has a watchdog timer built in. When no valid signal arrives at the ST pin of the DS1232 within the set cycle time, the RST and RST pins of the DS1232 will generate a reset signal to force the microprocessor to reset. This function is very effective in preventing the microprocessor from crashing due to interference and other reasons. The timing of the watchdog timer is determined by the TD pin of the DS1232, see Table 1 for details.

The periodic input signal ST of the watchdog timer can be obtained from the address signal, data signal or control signal of the microprocessor. No matter which signal is used, it must be able to periodically access the DS1232. For the MCS51 series microcontrollers, it is recommended to use the ALE signal.

3. DS1232 Typical Application Circuit

The typical interface circuit between DS1232 and MCS51 series microcontroller is shown in Figure 3.

4. Precautions for use

Although DS1232 has the feature of simple interface with microprocessor, the following points should be noted in its use:
(1) In addition to being connected to ALE of MCS51 microcontroller, ST can also be connected to other signal lines, but it must be ensured that the watchdog timer is reset before the watchdog timer count overflows. (2)
There is no pull-up resistor on the 6th pin inside DS1232. If other peripheral interface chips of the microcontroller need to use a low-level reset signal, then a pull-up resistor must be connected to this pin, as shown in R in Figure 3.
(3) If the user target board is debugged with an emulator and the ST terminal is connected to the ALE of the microcontroller, it is best not to insert the DS1232 chip first, because when the emulator is connected to the PC and the program is run in single-step, the ALE signal of the microcontroller is not continuously supplied, which can easily cause abnormal reset and affect the debugging work.