Technical data

Functional Description

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Most DRAM devices require a certain number of access cycles before the

DRAMs are fully operational. Normally this requirement is met by the

onboard refresh circuitry and normal DRAM initialization. However,

software should insure a minimum of 10 initialization cycles are

performed to each bank of RAM.

SRAM Options

The MVME172LX provides 128KB of 32-bit-wide onboard static RAM

in a single non-interleaved architecture with onboard battery backup. The

SRAM arrays are not parity protected.

The battery backup function for the onboard SRAM and the mezzanine

SRAM is provided by an Electro Marketing EM1275 device (or

equivalent) that supports primary and secondary power sources. In the

event of a main board power failure, the EM1275 checks power sources

and switches to the source with the higher voltage.

If the voltage of the backup source is lower than two volts, the EM1275

blocks the second memory cycle; this allows software to provide an early

warning to avoid data loss. Because the second access may be blocked

during a power failure, software should do at least two accesses before

relying on the data.

The MVME172LX provides jumpers (on J14) that allow either power

source of the EM1275 to be connected to the VMEbus +5V STDBY pin or

to one cell of the onboard battery. For example, the primary system backup

source may be a battery connected to the VMEbus +5V STDBY pin and

the secondary source may be the onboard battery. If the system source

should fail or the board is removed from the chassis, the onboard battery

takes over.

Caution

For proper SRAM operation, some jumper combination must

be installed on the Backup Power Source Select header (refer

to the jumper information in Chapter 1). If one of the jumpers

is set to select the battery, a battery must be installed on the

MVME172LX. The SRAM may malfunction if inputs to the

EM1275 are left unconnected.