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Advantages of Using a MicroMonitor and a Simple Application Demonstrating the

2008-9-24 17:04

<p><font face="Verdana">Advantages of Using a microMonitor and a Simple Application Demonstrating the Use of a DS1232</font></p>
<p><font face="Verdana">Introduction<br/>Dallas Semiconductor’s DS1232 MicroMonitor chip is a highly integrated solution to add power-on reset<br/>delay, a pushbutton reset controller, robust power failure monitoring, and watchdog timer functionality to<br/>your microprocessor system with the addition of a single chip. The chip also offers both active and low reset<br/>signals, and selectivity of the watchdog time out period and the voltage monitoring level. The high level of<br/>integration reduces both the cost of implementation and the board space required. The chips are available in<br/>several packages including 8-pin mSOP, SOIC, and DIP.<br/>Advantages of Using a Dallas Semiconductor DS1232 MicroMonitor<br/>Power-on Reset Delay Allows Board Resources to Stabilize Before Execution Begins<br/>Precise control of a microcontroller’s reset pin is important at all times during the operation of a circuit.<br/>Thus, it is imperative that the reset pin is controlLED during both normal operation and during the power-up<br/>time of the circuit. One common problem with many microprocessor circuits is that the microcontroller<br/>begins execution before the memory and I/O resources available to it have a stable power supply. Some chips<br/>require hundreds of milliseconds to reset themselves and ready for operation. IF a microprocessor begins<br/>execution within microseconds of a reset, it could be executing based on invalid input from the system<br/>resources. This can be resolved by adding a power-on reset (POR) circuit that holds the microprocessor in<br/>reset during the power-up sequence for 100s of milliseconds after a reset occurs. The DS1232 will hold a<br/>microcontroller’s reset pin active for a minimum time of 250ms (typically 610 ms.) Additionally, it can<br/>provide either an active high or active low reset, which makes it compatible with any microprocessor.<br/>Pushbutton Reset Circuit Debounces a Momentary Switch and Provides a Solid Reset Pulse<br/>Another common problem in microprocessor circuits is that a less than peRFect reset signal causes the<br/>microprocessor to reset several times, possibly executing some fragment of the initialization routine several<br/>times before the reset finally stabilizes. In some circuits this is trivial. However, in other circuits this could<br/>cause major system problems. The DS1232 has internal circuitry that debounces a push-button and provides<br/>the system a clean reset signal. This eliminates the need to have both a separate power-on reset circuit and<br/>pushbutton circuitry attached separately to the reset pin. Plus, the DS1232 will hold the reset pin low for at<br/>least 250 ms to guarantee that the manual reset is received correctly by the microprocessor every time.<br/>The DS1232 Provides Power Failure Monitoring<br/>Brownouts and power failures are a reality for most microprocessor systems. Since there is no way to avoid<br/>the occasional power cycle, a robust microprocessor solution must account for the various power failure<br/>modes. The DS1232 monitors the power supply to a microprocessor and forces the microprocessor into its<br/>reset state if there is a brownout or if the power fails altogether. This guarantees that the system will not try<br/>to operate during irregular supply voltage conditions, and it can prevent the microprocessor from executing<br/>code when it should really be waiting for the supply voltage to return to a valid state.</font></p>
<p><font face="Verdana">It<br/>Watchdog timers are used to ensure that if the code operating on a microprocessor enters into an<br/>unanticipated state, then the processor will reset after some minimal amount of time elapses. Many<br/>microprocessors have an internal watchdog timer that handles this function without an external component.<br/>However, the internal watchdog timer can be disabled by the pseudo-random code that is executing on the<br/>microprocessor. The external watchdog timer on the DS1232 cannot be disabled. This gives you peace of<br/>mind, knowing that if the MicroMonitor is not strobed, then the microcontroller will be reset. Another nice<br/>feature of the DS1232 is that the strobe period can be varied to one of three different values. The device thus<br/>works well with faster and slower microprocessors because you can program the watchdog timer to an<br/>appropriate rate for any microcontroller. Another advantage of the external watchdog timer is that the strobe<br/>signal is visible to the external world because it is on an I/O pin. This allows easier debugging of watchdogrelated<br/>problems.<br/>Hardware Required for Using a DS1232 with a Microcontroller<br/>The greatest asset of the DS1232 is that all of the functions listed above are implemented in a single package.<br/>This functionality does not require a complex chipset. The schematic below shows how the DS1232 is used<br/>with a Dallas Semiconductor ds87C520 Microcontroller. Since most systems incorporate a pushbutton reset,<br/>the only things added are the DS1232, a pull-up resistor for the active low output, and a decoupling capacitor<br/>to reduce the possibility of noise on the power supply causing a reset. The pull-up resistor is added here<br/>because it is used to generate the oscilloscope plots shown on the following pages. The RST output of the<br/>DS1232 is not required by the DS87C520. The LEDs on the diagram are used to signal when ISRs (Interrupt<br/>Service Requests) are being serviced.</font></p>
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