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STM706T Datasheet(PDF) 11 Page - STMicroelectronics |
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STM706T Datasheet(HTML) 11 Page - STMicroelectronics |
11 / 32 page STM706T/S/R, STM706P, STM708T/S/R Operation Doc ID 10518 Rev 12 11/32 3 Operation 3.1 Reset output The STM70x supervisor asserts a reset signal to the MCU whenever VCC goes below the reset threshold (VRST), a watchdog timeout occurs (if WDO is connected to MR), or when the push-button reset input (MR) is taken low. RST is guaranteed to be a logic low (logic high for STM706P and STM708T/S/R) for VCC < VRST down to VCC =1 V for TA = 0 °C to 85 °C. During power-up, once VCC exceeds the reset threshold an internal timer keeps RST low for the reset timeout period, trec. After this interval RST returns high. If VCC drops below the reset threshold, RST goes low. Each time RST is asserted, it stays low for at least the reset timeout period (trec). Any time VCC goes below the reset threshold the internal timer clears. The reset timer starts when VCC returns above the reset threshold. 3.2 Push-button reset input A logic low on MR asserts reset. Reset remains asserted while MR is low, and for trec (see Figure 27) after it returns high. The MR input has an internal 40 k Ω pull-up resistor, allowing it to be left open if not used. This input can be driven with TTL/CMOS-logic levels or with open-drain / collector outputs. Connect a normally open momentary switch from MR to GND to create a manual reset function; external debounce circuitry is not required. If MR is driven from long cables or the device is used in a noisy environment, connect a 0.1 µF capacitor from MR to GND to provide additional noise immunity. MR may float, or be tied to VCC when not used. 3.3 Watchdog input (STM706T/S/R and STM706P) The watchdog timer can be used to detect an out-of-control MCU. If the MCU does not toggle the watchdog input (WDI) within tWD (1.6 s), the watchdog output pin (WDO) is asserted. The internal 1.6s timer is cleared by either: 1. a reset pulse, or 2. by toggling WDI (high-to-low or low-to-high), which can detect pulses as short as 50 ns. See Figure 28 for STM706T/S/R and STM706P. The timer remains cleared and does not count for as long as reset is asserted. As soon as reset is released, the timer starts counting. Note: The watchdog function may be disabled by floating WDI or tri-stating the driver connected to WDI. When tri-stated or disconnected, the maximum allowable leakage current is 10 µA and the maximum allowable load capacitance is 200 pF. 3.4 Watchdog output (STM706T/S/R and STM706P) When VCC drops below the reset threshold, WDO will go low even if the watchdog timer has not yet timed out. However, unlike the reset output, WDO goes high as soon as VCC exceeds the reset threshold. WDO may be used to generate a reset pulse by connecting it to the MR input. |
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