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SP708S Datasheet(PDF) 10 Page - Exar Corporation |
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SP708S Datasheet(HTML) 10 Page - Exar Corporation |
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10 / 16 page  S SP P7 70 06 6R R/ /S S/ /T T -- S SP P7 70 08 8R R/ /S S/ /T T 3 3..0 0V V/ /3 3..3 3V V L Lo ow w P Po ow we err M Miiccrro op prro occe es ss so orr S Su up pe errv viis so orry y C Ciirrccu uiitts s © 2010 Exar Corporation 10/16 Rev. 2.0.0 FEATURES The SP706R/S/T and SP708R/S/T series provides four key functions: 1. A reset output during power-up, power- down and brownout conditions. 2. An independent watchdog output that goes LOW if the watchdog input has not been toggled within 1.6 seconds. 3. A 1.25V threshold detector for power-fail warning, low battery detection, or monitoring a power supply other than +3.3V/+3.0V. 4. An active-LOW manual-reset that allows RESET to be triggered by a pushbutton switch. The SP706R/S/T devices are the same as the SP708R/S/T devices except for the active- HIGH RESET substitution of the watchdog timer. THEORY OF OPERATION The SP706R/S/T-SP708R/S/T series is a microprocessor (µP) supervisory circuit that monitors the power supplied to digital circuits such as microprocessors, microcontrollers, or memory. The series is an ideal solution for portable, battery-powered equipment that requires power supply monitoring. Implementing this series will reduce the number of components and overall complexity. The watchdog functions of this product family will continuously oversee the operational status of a system. The operational features and benefits of the SP706R/S/T-SP708R/S/T series are described in more detail below. RESET OUTPUT A microprocessor's reset input starts the µP in a known state. The SP706R/S/T-SP708R/S/T series asserts reset during power-up and prevents code execution errors during power down or brownout conditions. On power-up, once VCC reaches 1V, RESET is a guaranteed logic LOW of 0.4V or less. As VCC rises, RESET stays LOW. When VCC rises above the reset threshold, an internal timer releases RESET after 200ms. RESET pulses LOW whenever VCC dips below the reset threshold, such as in a brownout condition. When a brownout condition occurs in the middle of a previously initiated reset pulse, the pulse continues for at least another 140ms. On power down, once VCC falls below the reset threshold, RESET stays LOW and is guaranteed to be 0.4V or less until VCC drops below 1V. The active-HIGH RESET output is simply the complement of the RESET output and is guaranteed to be valid with VCC down to 1.1V. Some µPs, such as Intel's 80C51, require an active-HIGH reset pulse. WATCHDOG TIMER The SP706R/S/T-SP708R/S/T watchdog circuit monitors the µP's activity. If the µP does not toggle the watchdog input (WDI) within 1.6 seconds and WDI is not tri-stated, WDO goes LOW. As long as RESET is asserted or the WDI input is tri-stated, the watchdog timer will stay cleared and will not count. As soon as RESET is released and WDI is driven HIGH or LOW, the timer will start counting. Pulses as short as 50ns can be detected. Typically, WDO will be connected to the non- maskable interrupt input (NMI) of a µP. When VCC drops below the reset threshold, WDO will go LOW whether or not the watchdog timer had timed out. Normally this would trigger an NMI but RESET goes LOW simultaneously and thus overrides the NMI. If WDI is left unconnected, WDO can be used as a low-line output. Since floating WDI disables the internal timer, WDO goes LOW only when VCC falls below the reset threshold, thus functioning as a low-line output. Fig. 18: Watchdog Timing Waveforms |
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