The code described in this application note should be used for all versions of the 71M6521, including the 71M6521BE, 71M6521DE, and 71M6521FE. Older codes contained RTC code that integrated long term jitter into the RTC's drift correction after reset. In some instances, operating with an improperly initialized internal power supply for an extended period of time can corrupt the RTC.Įliminating all EMI in a meter is not practical, but firmware can detect and avoid the above issues by testing the trim fuses of the IC when it enters brownout mode.Īnother issue addressed by this new demo code version is faulty RTC drift correction. This failure could cause the external power supply voltage applied to the V1 pin to be misread, causing an unexpected transition to sleep mode when the wake timer has not been set. Unexpected changes to internal power supply voltages and detection thresholds.Depending on the firmware, the unpredictable software execution can in some instances cause the internal clock to stop, preventing the watchdog timer from resetting the meter. IC failures caused by a misread of the trim fuses can cause various symptoms, depending on which trim fuse read is affected. Failures based on trim fuse misreads are extremely rare and can only be seen when large numbers of meters are power-cycled over days or weeks. This oscillation causes the 71M6521 to switch rapidly and repeatedly between regular current consumption (mission mode) and low current consumption (brownout mode). The likelihood of potential failures is increased when the power supply rapidly "oscillates" the 71M6521 between mission and brownout modes, which can occur when the AC mains voltage is ramped up or down slowly in conjunction with a "soft" power supply. If this occurs, a number of failures can result. In the low-power modes, regular operating currents are not available to the IC.Įxtreme electromagnetic interference (EMI) or severe and repeated disruptions of the supply voltage can sometimes affect the trim read/refresh cycle, resulting in the IC misreading one of its trim fuses. The 71M6521 ICs support three low-power modes (sleep mode, LCD only mode, and brownout mode). In the 71M6521 family, a total of seven individual trim fuse values can be read by first writing any number from 1 to 7 to I/O RAM register 0x20FD ( TRIMSEL) and then reading the corresponding fuse value from I/O RAM register 0x20FF ( TRIM).įigure 1. Some examples are the fuses used to trim the V REF voltage to within ☑mV of 1.195 VDC and the fuses used to trim the V BIAS internal voltage to the target value.ĭuring operation of the IC, the contents of the trim fuses are read and refreshed by hardware on the IC at regular intervals, and the values obtained by the trim fuse read are provided to both the hardware controlled by the fuse values and to the fuse register locations in I/O RAM (see Figure 1). After the production stage, the contents of the trim fuses cannot be changed. Trim fuses are a form of nonvolatile (NV) memory used in the production of Maxim's Teridian energy metering ICs to adjust analog and digital characteristics of the ICs. This document isolates the changes and describes details so that the updates can be retrofitted into customer firmware, if needed. These improvements make transitions between power modes on the 71M6521 more reliable and support proper correction of the real-time clock (RTC) after return to mission mode. This document describes two firmware improvements that were applied to the Demo Code version 4.8p for Maxim's Teridian ™ 71M6521 family of energy meter ICs.
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