Specifications
Hardware Configuration
2–8
(To connect the on-board battery to byte-wide socket S0 to back up an SRAM, install jumpers on W2
and W21-1/2. If you use another type of memory device in S0, you must remove W2 and install a
jumper on W21-2/3. )
Here is the formula for calculating battery life (in hours):
Battery life = (165 milliamp-hours
÷
(1 uA + SRAM backup current)) × Duty Cycle
If the memory device you select draws too much current from the onboard battery, you can add an
external 3.6 volt lithium battery to J16. Connect the positive terminal to J16-15 and the negative
terminal to J16-16. (To conform to UL recommendations, internal and external batteries are
connected by Schottky diodes using a circuit in which two blocking devices are in series with the
batteries.)
2.2.4 Cooling Requirements
The 486 CPU, DRAM, video controller, and core logic chip draw most of the power and generate
most of the heat. The board uses a 3.3 volt version of the 486 CPU chip to minimize power drain. In
addition, a heat sink is provided for the CPU. However, you must provide additional air flow to keep
the board’s 486DX within its specified case temperature of 85°C when the ambient temperature is
high.
2.3 CPU CLOCK AND CACHE CONFIGURATION
The LB/486i is factory configured to run at the maximum speed allowed by the CPU option. CPUs
with internal clock rates of 66 MHz and 100 MHz can only be used at those speeds. The 133 MHz
version can be jumpered to run at 100 MHz using W22. The slower clock rate can be used to reduce
power requirements and improve thermal characteristics. The CPU manufacturer’s datasheet gives a
guideline of 7 mA / MHz for maximum current requirements. Table 2-5 indicates the function of
W22 in setting the internal clock speed. The shaded field indicates factory default.
W22 ON W22 OFF
133 MHz 100 MHz
Table 2-5 CPU Speed Settings (W22)
Another jumper, W10, is used to control the configuration of the CPU’s internal Cache. This
16 Kbyte Cache is factory configured to ‘Write-Back’. This setting can be changed to
‘Write-Through’ by moving the shunt on W10 to pin 2/3.