Phytec miniMODUL-537, Hardware Manual

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RAM-Write-Protection and Battery Back-up
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PHYTEC Meßtechnik GmbH 1993 L-006e_2
43
7 RAM-Write-Protection and Battery Back-up
Write-protection and battery back-up are achieved by U3, U6, and
Q2..Q4. An external battery must be connected to A42. The current
consumption from VPD depends on the type of RAM and their ex-
tensions. Installing a good 32 kByte RAM (1µA) a 100 mAh battery
will do for several years.
The RAMs U12 and U13 and the optional Real-Time Clock U14
(RTC 72421) are supplied by the battery as soon as VCC decreases
below 4.6 or 4.8 V. U12 is either supplied with VCC or VPD. This is
adjusted by jumper JP12. If an EEPROM is installed, jumper JP12
must be adjusted to VCC. A RAM in U12 can be supplied by VCC
or VPD.
If you forget to set jumper JP12 correctly, the battery will be
discharged quickly if an EEPROM is installed in U12.