# Self Testing in Embedded Systems

- Link: https://studyelectrical.com/2021/07/self-testing-in-embedded-systems.html
- Published: 2021-07-09T11:32:26+00:00
- Author: Tesla

The implementation of some self-testing capabilities in an embedded design is worthwhile
unless available memory is completely exhausted. 

Although modern electronics are amazingly reliable, they are still susceptible to
failure. Failures in embedded systems can be broadly divided into four categories:

 1. CPU
 2. Peripheral
 3. Memory
 4. Software errors 

If a CPU fails, it tends to be a hard failure. There is no possibility for self-
testing in this system. Partial failure of a CPU is very unlikely. When there are
multiple cores in a system, it is good practice to assign one core as “master” so
that it can monitor the system integrity. 

![Moving ones test](https://studyelectrical.com/wp-content/uploads/2021/07/Moving-
ones-test.jpg)

Moving Ones Test

Peripherals can fail in a variety of ways, but many of them are device/application
specific. When a device fails to respond to its address, the trap occurs; it is 
essential to include a trap handler to handle this fault. Otherwise, communications
devices commonly include a “loopback” mode that enables testing of transmission 
and reception and associated interrupts. 

Memory failures can happen at any time. This failure may be transient – for example,
a single bit being flipped by a passing cosmic ray. Such a fault is generally not
detectable and may result in a software crash. Therefore, it is essential to accommodate
crash recovery. There could be a hard failure if the address response is not received,
or if bits are stuck at 0 or 1. A trap handler deals with the former, but the latter
requires some specific testing. Comprehensive memory testing can only be done on
device start-up. A Moving Ones test is effective. 

![stack guard words](https://studyelectrical.com/wp-content/uploads/2021/07/stack-
guard-words.jpg)

Stack guard words

While the device is operating, pattern testing can be performed on individual bytes
or words which may reveal certain types of failures. 

In modern devices, software is the most complex component. Even though software 
does not wear out, its complexity can result in faults that are difficult to detect
during development. Good, defensive coding techniques can help anticipate some problems.

In general, there are two types of software errors:

 1. data corruption
 2. code looping. 

Data corruption can be caused by pointer misusage, which is hard to detect or prevent,
but it can also be as a result overflow of a data structure, like an array or the
stack. The insertion of “guard words” can assist with the detection of overflow 
before any damage is caused. 

Code looping can be addressed by careful design – precautions like timeouts on waiting
for devices – or some kind of watchdog facility (in hardware or software) that traps
unresponsive code.

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