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ASM706CUA Datasheet(PDF) 6 Page - ON Semiconductor |
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ASM706CUA Datasheet(HTML) 6 Page - ON Semiconductor |
6 / 13 page ASM705, ASM706, ASM707, ASM708, ASM813L http://onsemi.com 6 Detailed Description A proper reset input enables a microprocessor / microcontroller to start in a known state. ASM70X and ASM813L assert reset to prevent code execution errors during power−up, power−down and brown−out conditions. RESET/RESET Timing The RESET/RESET signals are designed to start a mP/mC in a known state or return the system to a known state. The ASM707/708 have two reset outputs, one active−HIGH RESET and one active−LOW RESET output. The ASM813L has only an active−HIGH output. RESET is simply the complement of RESET. RESET is guaranteed to be LOW with VCC above 1.2 V. During a power−up sequence, RESET remains low until the supply rises above the threshold level, either 4.65 V or 4.40 V. RESET goes high approximately 200 ms after crossing the threshold. During power−down, RESET goes LOW as VCC falls below the threshold level and is guaranteed to be under 0.4 V with VCC above 1.2 V. In a brownout situation where VCC falls below the threshold level, RESET pulses low. If a brown−out occurs during an already initiated reset, the pulse will continue for a minimum of 140 ms. Power Failure Detection with Auxiliary Comparator All devices have an auxiliary comparator with 1.25 V trip point and uncommitted output (PFO) and noninverting input (PFI). This comparator can be used as a supply voltage monitor with an external resistor voltage divider. The attenuated voltage at PFI should be set just below the 1.25 threshold. As the supply level falls, PFI is reduced causing the PFO output to transit LOW. Normally PFO interrupts the processor so the system can be shut down in a controlled manner. Figure 3. WDI Three−state Operation Manual Reset (MR) The active−LOW manual reset input is pulled high by a 250 mA pull−up current and can be driven low by CMOS/TTL logic or a mechanical switch to ground. An external debounce circuit is unnecessary since the 140 ms minimum reset time will debounce mechanical pushbutton switches. By connecting the watchdog output (WDO) and MR, a watchdog timeout forces RESET to be generated. The ASM813L should be used when an active−HIGH RESET is required. Watchdog Timer The watchdog timer available on the ASM705/706/813L monitors mP/mC activity. An output line on the processor is used to toggle the WDI line. If this line is not toggled within 1.6 seconds, the internal timer puts the watchdog output, WDO, into a LOW state. WDO will remain LOW until a toggle is detected at WDI. If WDI is floated or connected to a three−stated circuit, the watchdog function is disabled, meaning, it is cleared and not counting. The watchdog timer is also disabled if RESET is asserted. When RESET becomes inactive and the WDI input sees a high or low transition as short as 50 ns, the watchdog timer will begin a 1.6 second countdown. Additional transitions at WDI will reset the watchdog timer and initiate a new countdown sequence. WDO will also become LOW and remain so, whenever the supply voltage, VCC, falls below the device threshold level. WDO goes HIGH as soon as VCC transitions above the threshold. There is no minimum pulse width for WDO as there is for the RESET outputs. If WDI is floated, WDO essentially acts as a low−power output indicator. Figure 4. Watchdog Timing |
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