NOVRAM AUTOSTORE Considerations ® Application Note April 27, 2005 AN56.0 Author: Richard Downing Introduction and is derived by taking the integral of: Intersil’s NOVRAM (Nonvolatile RAM) devices are highspeed SRAMs overlaid bit-for-bit with a nonvolatile E2PROM array. The AUTOSTORE (AS) feature available on these parts automatically saves the RAM contents to E2PROM during power-down. AS typically requires some basic Vcc holdup circuitry to ensure successful completion. dV i = C ------dt To use the circuit with a 5V supply requires that diode DH to have low forward voltage drop VF. A Schottky diode such as the 1N5817 (available from Motorola or Philips) is suitable with maximum VF of 0.32V at 100mA. The cost of the component is around 20¢ in volume. Basic Autostore Circuit The minimal components necessary to achieve an AS are a capacitor and diode which connect to the NOVRAM as shown in Figure 1. The “hold-up” capacitor must be capable of supplying the maximum AS current (ICC3 max.) for the maximum AS period (TASTO max.) as Vcc falls to a level between the AS threshold voltage (VASTH min.) and the AS end voltage (VASEND min.). The “hold-up” capacitor value (CH) is calculated using the equation: Power Supply Limitation Intersil specifies a NOVRAM Vcc operating range between 4.5V and 5.5V. The diode in the AS circuit, however, results in a voltage drop (VF) between the power supply and the NOVRAM’s Vcc pin. A maximum VF of 0.32V results in a possible AS at a supply voltage of 4.62V. For a reasonable margin of safety, the suggested AS circuit should be powered using a supply that operates above 4.75V. A 5V ±5%. power supply would be one way of ensuring correct AS operation. ( I CC3 max ) ( T ASTO max ) C H = -------------------------------------------------------------------V ASTH min – V ASEND max TABLE 1. NOVRAM AUTOSTORE DEVICE SUMMARY VASTH (V) VASEND (V) TASTO (ms) ICC3 (mA) CH (µF) Max Min Max Max Min Part No. Min X24C45 4.0 4.3 3.5 5.0 2.0 20 X20C16 4.0 4.3 3.5 2.5 2.5 13 VCC VCC VCC 1N5817 X20C16 VCC DH NE VCC VCC VCC OE 1N914 CE + VCC 100µF 0.1µF CE1 CE VBAT CE OUT CE IN CH µC LT691 NOVRAM FIGURE 2 FIGURE 1 1 CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures. 1-888-INTERSIL or 1-888-352-6832 | Intersil (and design) is a registered trademark of Intersil Americas Inc. Copyright Intersil Americas Inc. 2005. All Rights Reserved All other trademarks mentioned are the property of their respective owners. Application Note 56 Pre-AUTOSTORE Data Corruption Conclusion If the supply to a microcontroller interfacing to a parallel NOVRAM is lost while the NOVRAM remains powered, inadvertent writes to the SRAM can occur. The uncertain state of the microcontroller control lines during power-down may cause fluctuations in the status of CE, WE, and OE, which can result in a write condition. Invalid SRAM data may be transferred to the nonvolatile E2PROM array if these changes occur before an AS. These inadvertent write conditions can be avoided using supervisory ICs available from manufacturers such as Linear Technology or Maxim. These devices control the CE of the NOVRAM so that below a set supply threshold voltage, CE is held HIGH and the NOVRAM is placed into a standby condition. An example of such a circuit is shown in Figure 2. NOVRAMs combine the advantages of fast access RAMs and nonvolatility for E2PROMs. The AUTOSTORE feature enables data integrity to be maintained during power-down. In most applications two additional components (a diode and capacitor) are all that is needed to ensure successful completion of an AUTOSTORE. A supervisory IC may also be useful to protect the part from inadvertent writes. Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is cautioned to verify that the Application Note or Technical Brief is current before proceeding. For information regarding Intersil Corporation and its products, see www.intersil.com 2 AN56.0 April 27, 2005