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ST7LITE0 データシート(PDF) 47 Page - STMicroelectronics |
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ST7LITE0 データシート(HTML) 47 Page - STMicroelectronics |
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47 / 122 page  ST7LITE0, ST7SUPERLITE 47/122 LITE TIMER (Cont’d) 11.1.3 Functional Description The value of the 8-bit counter cannot be read or written by software. After an MCU reset, it starts incrementing from 0 at a frequency of fOSC/32. A counter overflow event occurs when the counter rolls over from F9h to 00h. If fOSC = 8 MHz, then the time period between two counter overflow events is 1 ms. This period can be doubled by set- ting the TB bit in the LTCSR register. When the timer overflows, the TBF bit is set by hardware and an interrupt request is generated if the TBIE is set. The TBF bit is cleared by software reading the LTCSR register. 11.1.3.1 Watchdog The watchdog is enabled using the WDGE bit. The normal Watchdog timeout is 2ms (@ = 8 MHz fOSC), after which it then generates a reset. To prevent this watchdog reset occuring, software must set the WDGD bit. The WDGD bit is cleared by hardware after tWDG. This means that software must write to the WDGD bit at regular intervals to prevent a watchdog reset occurring. Refer to Fig- ure 31. If the watchdog is not enabled immediately after reset, the first watchdog timeout will be shorter than 2ms, because this period is counted starting from reset. Moreover, if a 2ms period has already elapsed after the last MCU reset, the watchdog re- set will take place as soon as the WDGE bit is set. For these reasons, it is recommended to enable the Watchdog immediately after reset or else to set the WDGD bit before the WGDE bit so a watchdog reset will not occur for at least 2ms. Note: Software can use the timebase feature to set the WDGD bit at 1 or 2 ms intervals. A Watchdog reset can be forced at any time by setting the WDGRF bit. To generate a forced watchdog reset, first watchdog has to be activated by setting the WDGE bit and then the WDGRF bit has to be set. The WDGRF bit also acts as a flag, indicating that the Watchdog was the source of the reset. It is au- tomatically cleared after it has been read. Caution: When the WDGRF bit is set, software must clear it, otherwise the next time the watchdog is enabled (by hardware or software), the micro- controller will be immediately reset. Hardware Watchdog Option If Hardware Watchdog is selected by option byte, the watchdog is always active and the WDGE bit in the LTCSR is not used. Refer to the Option Byte description in the "device configuration and ordering information" section. Using Halt Mode with the Watchdog (option) If the Watchdog reset on HALT option is not se- lected by option byte, the Halt mode can be used when the watchdog is enabled. In this case, the HALT instruction stops the oscilla- tor. When the oscillator is stopped, the Lite Timer stops counting and is no longer able to generate a Watchdog reset until the microcontroller receives an external interrupt or a reset. If an external interrupt is received, the WDG re- starts counting after 256 CPU clocks. If a reset is generated, the Watchdog is disabled (reset state). If Halt mode with Watchdog is enabled by option byte (No watchdog reset on HALT instruction), it is recommended before executing the HALT instruc- tion to refresh the WDG counter, to avoid an unex- pected WDG reset immediately after waking up the microcontroller. 1 |
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