DSP 104-plus, Reference Manual

Page: 130 / 140

H2 164004.C03
H.1.3 Failure Effect Assumptions
The assumption is made that each and every component failure mode will result in the
failure of the TP400. This is clearly a conservative assumption. Many component
failure modes, for example a 25% drift in the value of a pull-up resistor, are unlikely to
have any effect on the operation of the device.
H.2 RELIABILITY DATA USEAGE
H.2.1 Operating Temperature
This is the operating ambient temperature for the TP400, in degrees Celsius. This
should be the in-use steady state temperature as measured by the on-board
temperature sensor. Since the sensor is located under the Geode GX1, the hottest
component on the assembly, using this temperature against Table H1 and Figures H1
and H2 will provide a conservative estimate of the reliability of the TP400.
On examining the reliability derating curves produced for this Appendix, it is clear that
thermal management is important in the application of the TP400. Where reliance is
placed on a cooling component/system for the thermal management of the TP400, the
cooling component/system reliability will need to be assured.
Note that the Tables and Figures range from 25ºC to 75ºC.
H.2.2 Flash Memory Usage
It should be noted that the Flash memory in the TP400 is expected to withstand
1,000,000 write/erase cycles per sector. If very frequent file management and update
tasks, using the Flash memory for storage, are implemented within the application, an
estimate of the write/erase cycle frequency on the reliability of the TP400 will have to
be calculated over the expected life of the unit. For example, if each sector of the flash
memory were written/erased every 5 minutes, the device life would expire after 9.5
years.
If the Flash memory is used purely for BIOS storage, this limitation has no effect. Read
cycles have no effect on the longevity of the Flash Memory.
H.2.3 Usage Environment
This is the operating environment of the TP400. The following list gives the
environment code and description for the various Bellcore environments:
Benign (fixed and controlled): Nearly zero environmental stress with the optimum
engineering operation and maintenance. Typical applications are environmentally
controlled control rooms and environmentally controlled customer premises. Not
analogous to any marine environment. It should be noted that this usage environment
is only achievable by very careful thermal management of a small, thermally active unit
like the TP400.
Fixed (fixed and uncontrolled): Some environmental stress with limited maintenance.
Typical applications are manholes, poles, remote terminals, and customer premise