The Circuit Designer's Companion - diagramas.diagram...

Digital circuits 219not to be over-enthusiastic with the spikes, or you may end up destroying several goodprototypes. Build up a large enough statistical base of events to have a good chance ofcovering all operational conditions, and check that after each one the processor hasbeen correctly reset and has recovered to normal operation. (This is a good task for ajunior technician.) An LED on the watchdog output is useful to detect its barks. Payparticular attention to what happens when you apply a burst of spikes, so that theprocessor is hit again just as it is recovering from the last one. This is a vulnerablecondition but unhappily it is a common occurrence in practice.As well as testing the reliability of the watchdog, remember to include a link todisable it so that you can test new versions of software.6.4.3 Supervisor designThe traditional method for power-on reset is the simple R-C network across the powerrail (Figure 6.32(a)). This circuit delays the voltage rise at the RESET input for a giventime, long enough for the micro’s required start-up period. The diode, shown dottedhere, is needed to discharge the capacitor rapidly in the event of a short V CCinterruption. Even so, the circuit is susceptible to interruptions or dips of up to a fewmilliseconds, when the capacitor cannot discharge fast enough to bring the RESETinput below its required threshold. It also depends on a minimum rate of rise of V CC ,which may not be achieved in all circumstances, and it gives no early warning of animpending power failure. This simple approach suits consumer and gadgetapplications, where processor unreliability is no more than mildly inconvenient.The undervoltage detector of Figure 6.32(b) offers an improvement, whereby theRESET capacitor is held low until the input voltage to the regulator has reached asufficiently high value for the regulator output to be stable. This point is set by R1 andR2. Any transient undervoltage will cause the comparator to rapidly discharge thecapacitor and generate a reset, and the micro will also be forced to reset reliably as soonas the power fails. This still does not provide a power fail early warning, for whichanother comparator is needed as at Figure 6.32(c).V CCV inRegV CCR1RESETR2RESETa) simple reset b) undervoltage resetV inc) combinedundervoltage resetand power fail detectABRegLow lineMonopower failRESETNMIV CCFigure 6.32 Microprocessor reset circuits