Class B Requirements

For an appliance to comply with the Class B requirements, the control software must detect and handle the faults specified for the system components in Table 1. In addition to these tests and Fault detections, the software must be properly documented to pass a certification. This includes the program sequence, control and data flow, timing, Fault tree, and general design philosophy.

Table 1. Short Overview of Components and Faults/Errors for Test (Table H.11.12.7)
Component Component to Test Typical Error Detected by Tests
1 CPU -
1.1 Registers Stuck at
1.3 Program counter Stuck at
2 Interrupt handling and execution No/too frequent interrupts
3 Clock Wrong frequency
4 Memory(1) -
4.1 Invariable memory All single bit faults
4.2 Variable memory DC Fault
4.3 Addressing Stuck at
5 Internal data path(1) -
5.1 Data Stuck at
5.2 Addressing Wrong address
6 External communication -
6.1 Data Too long Hamming distance
6.3 Timing Wrong point in time
7 I/O peripherals -
7.1 Digital I/O Fault conditions specified in H.27.1(2)
7.2.1 A/D- and D/A-converter Fault conditions specified in H.27.1(2)
7.2.2 Analog multiplexer Wrong addressing
9 Custom chips (ASIC, GAL, Gate array, etc.) Any output outside static and dynamic functional specifications
Note:
  1. 1.These are essentially the same for AVRs since SRAM, Flash, and EEPROM are all internal.
  2. 2.This table lists various external components and indicates whether a short and/or open Fault must be detected.

Several of these tests are inevitably application dependent. As an example, for I/O peripherals it is required to do plausibility checking of the input/output signals. This, in turn, depends on both the application and its implementation. The firmware library supplied with this application note, therefore, cannot cover all of the requirements in Table 1.