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908E621 Datasheet(PDF) 46 Page - Freescale Semiconductor, Inc |
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908E621 Datasheet(HTML) 46 Page - Freescale Semiconductor, Inc |
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46 / 60 page  Analog Integrated Circuit Device Data 46 Freescale Semiconductor 908E621 FUNCTIONAL DEVICE OPERATION LOGIC COMMANDS AND REGISTERS FACTORY TRIMMING AND CALIBRATION To enhance the ease-of-use of the 908E621, various parameters (e.g. ICG trim value) are stored in the flash memory of the device. The following flash memory locations are reserved for this purpose and might have a value different from the “empty” ($FF) state: • $FD80:$FDDF Trim and Calibration Values • $FFFE:$FFFF Reset Vector In the event the application uses these parameters, one has to take care not to erase or override these values. If these parameters are not used, these flash locations can be erased and otherwise used. Trim Values The usage of the trim values located in the flash memory are explained through the following: Internal Clock Generator (ICG) Trim Value The internal clock generator (ICG) module is used to create a stable clock source for the microcontroller without using any external components. The untrimmed frequency of the low frequency base clock (IBASE), will vary as much as ±25 percent due to process, temperature, and voltage dependencies. To compensate these dependencies, an ICG trim value is located at address $FDC2. After trimming, the ICG has a typ. range of ±2% (±3% max.), at nominal conditions (filtered (100nF), stabilized (4.7 μF) VDD = 5.0 V, TAMBIENT~25 °C), and will vary over temperature and voltage (VDD) as indicated in the 68HC908EY16 datasheet. To trim the ICG, this value has to be copied to the ICG Trim Register ICGTR at address $38 of the MCU. Important: The value must be copied after every reset. Watchdog Period Range Value (AWD Trim) The window watchdog supervises device recovery (e.g. from code runaways). The application software has to clear the watchdog within the open window. Due to the high variation of the watchdog period, and therefore the reduced width of the watchdog window, a value is stored at address $FDCF. This value classifies the watchdog period into 3 ranges (Range 0, 1, 2). This allows the application software to select one of three time intervals to clear the watchdog, based on the stored value. The classification is done, so that the application software can have up to ±19% variations of the optimal clear interval (e.g. caused by ICG variation). Effective Open Window Having a variation in the watchdog period in conjunction with a 50% open window, results in an effective open window, which can be calculated by: latest window open time: t_open = t_wd max / 2 earliest window closed time: t_closed = t_wd min Optimal Clear Interval The optimal clear interval, meaning the clear interval with the biggest possible variation to latest window open time, and to the earliest window closed time, can be calculated with the following formula: t_opt = t_open + (t_open+t_closed) / 2 See Table 13 to select the optimal clear interval for the watchdog based on the Window No. and chosen period. Table 13. Window Clear Interval Window Range Period Select bits Watchdog Period t_wd Effective Open Window Optimal Clear Interval $FDCF WDP1:0 min. max. Unit t_open t_closed Unit t_opt Unit max. variation 0 00 68 92 ms 46 68 ms 57 ms ±19.3% 01 34 46 23 34 28.5 10 17 23 11.5 17 14.25 11 8.5 11.5 5.75 8.5 7.125 1 00 92 124 ms 62 92 ms 77 ms ±19.5% 01 46 62 31 46 38.5 10 23 31 15.5 23 19.25 11 11.5 15.5 7.75 11.5 9.625 2 00 52 68 ms 34 52 ms 43 ms ±20.9% 01 26 34 17 26 21.5 10 13 17 8.5 13 10.75 11 6.5 8.5 4.25 6.5 5.375 |
Similar Part No. - 908E621_12 |
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Similar Description - 908E621_12 |
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