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Non-Volatile Ram Protection Circuit

IP.com Disclosure Number: IPCOM000035274D
Original Publication Date: 1989-Jun-01
Included in the Prior Art Database: 2005-Jan-28
Document File: 3 page(s) / 40K

Publishing Venue

IBM

Related People

Cecchi, DR: AUTHOR [+2]

Abstract

This circuit provides protection of NVRAM or EEPROM contents between system power-on and issuance of a power-on-reset signal. An otherwise unused voltage input is used as a control signal to prohibit the unintentional overwriting of vital product data.

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Non-Volatile Ram Protection Circuit

This circuit provides protection of NVRAM or EEPROM contents between system power-on and issuance of a power-on-reset signal. An otherwise unused voltage input is used as a control signal to prohibit the unintentional overwriting of vital product data.

The power-on-reset (POR) signal of certain computer systems is not a true POR in the usual definition of the signal. Normally one expects a POR signal to be active (logic low) from the time of power on until all voltage levels are up and within specifications. In some cases, however, the power may be applied for as long as several seconds before the POR signal to the logic cards becomes active. This creates a 'Window of Vulnerability' for a NVRAM because its control pins, driven by uninitialized logic, may have powered up in a state which will cause the NVRAM to initiate a store cycle, overwriting its contents with random data. To avoid this data loss, a circuit was devised which insures that a store operation cannot take place in the field, while maintaining the ability to load the NVRAM during card manufacture.

This circuit enables NVRAM STORE operations at the card tester by applying a voltage condition which will never be encountered in system operation.

The particular card this circuit was designed for does not use the + 12 volts supplied by the tester and system. The tester can drive this voltage as high as 15 volts, while in the system, it is supplied by a +/- 5% power supply, the result being that the highest voltage the card will ever see in system operation is 12 + 5% or 12.6 volts. Therefore,

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a circuit was designed which enables the NVRAM STORE operation only when the + 12 volt supply is at 13 volts or higher. This circuit can be modified for use with other voltage supplies by changing the value of a few components.

Fig. 1 shows the operation of the basic protection circuit. The 12-volt supply input is passed through two reverse biased 6.1 volt precision zener diodes (ZD1 and ZD2), dropping the voltage by 12.2 volts. This reduced voltage is then halved by a voltage divider consisting of two resistors of equal value (R1 and R2). This halved voltage is connected to the base of a...