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Application Note
NOVRAM AUTOSTORE Considerations
Introduction Xicor'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. 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: ( I CC3 max ) ( T ASTO max ) C H = -------------------------------------------------------------------V ASTH min - V ASEND max Table 1. NOVRAM AUTOSTORE Device Summary VASTH (V) Part No. X24C45 X20C16 Min 4.0 4.0 Max 4.3 4.3 VASEND (V) Min 3.5 3.5 TASTO (ms) Max 5.0 2.5 ICC3 (mA) Max 2.0 2.5 and is derived by taking the integral of: dV i = C -----dt
AN56
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. Power Supply Limitation Xicor 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.
CH (F) Min 20 13
Figure 1.
Figure 2.
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Xicor Application Note
Pre-AUTOSTORE Data Corruption 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.
AN56
Conclusion 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.
AN56-2

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