Data processing medium, its backup circuit and data processing system

There is disclosed a backup circuit for a memory card where the stored data in an S-RAM is retained even when the main battery is replaced. A backup circuit 16 for a memory card 2 contains a main battery 18 whose output voltage is 3.3 V. an auxiliary battery 20, and a charging prevention diode 22. The main battery 18 and the auxiliary battery 20 are connected in parallel. Between their junction 28 and a built-in power supply 8 (the output voltage of 5 V ) of a personal computer 4, the charging prevention diode 22 is provided. Because the second diode 22 is placed in a forward direction running from the junction 28 toward a voltage supply line 10, the auxiliaty battery 20 is charged only from the main battery 18, and a charging current from the built-in power supply 8 does not flow into the auxiliary battery 20. Therefore, the withstand voltage of the auxiliary battery 22 has only to be as high as the output voltage of the main battery 14. Accordingly, it is not necessary to take into account the withstand voltage of the auxiliary battery 22 with respect to the output voltage of the built-in battery 8.