What are embedded systems?

application specific

information processing system

which is embedded into

a bigger technical context or technical system

What do embedded systems consist of?

cooperating

optimized

hardware and software components

Are embedded systems majorly different from general purpose computing systems? (servers, pcs)?

yes

-> tasks differ substantially

What points in time are there for a real-time ES?

r_s -> release time

point in time where job becomes available for execution by the ES ( i.e. sensor data arrives…)

t_r -> response time

time from release time until embedded system finishes and responds

r_d -> (relative) deadline

time the ES has left (from the release time on) to respond

t_exe -> execution time (between r_s, r_s+t_r

real time:

t_r < t_d

What does the abbreviation RAS stand for?

reliability

availability

serviceability

How is safety defined?

failure of an embedded system

has no catastrophic consequence

for technical context

in higher-order system

What means and is the difference between fail-safe and fail-operational?

fail safe:

in face of failure or malfunction

-> shut down or go into safe state

fail operaional:

in face of failure or malfunciton

-> is able to continue operation (i.e. safety critical system like aircraft control…)

In what state of the development cycle should one consider RAS?

must not be added as afterthought

-> design it into the system from the very beginning

How is reliability (idea wise) defined?

R(t): Probability that system works correct until time t

! presuming it worked correct at reference time t0 = 0

What different types of changes in failure rate exist?

early “infant mortality” failure

wear out failure

constant (random) failure

=> stacks / adds up to

observed failure rate

What different durations/metrics exist in availability?

MTTR: mean time to repair

how long to go after failure back to operational

MTTF:

how long after last failure till next failure occurs

MTBF:

how long since (initial) occurence of last failure till next occurs?

How is availability defined?

A: fraction of time the system works correct in between two consecutive failures

How is serviceability measured?

S: measure considering the time it takes to repair system after a benign faiulre

-> MTTR

How does reliability of two parts of a system stack when they both need to work correct for the system to be operational?

add both lambda -> lambda_sys = lambda_1 + lambda_2

MTTF_sys = 1/(lambda_1 + lambda_2)

…

=> if vastly different -> reliability gets dominated by lowest MTTF (largest lambda)

How can one calculate the expected time of a system running with a failure probability lower of e.g. 10%?

need to find t until reliability is 90%

-> F(t) = 1 - R(t) (failure probability 10%…)

-> R(t) !>= 90%

-> e^(lambda_sys*t) >= 90%

ln(e^(lambda_sys*t)) >= ln(90%)

-(lambda_sys*t) >= ln(0.9)

t <= ln(0.9) / lambda_sys

How to calc the failure rate in a system with more than one component?

system failure rate is the sum of the individual failure rates

-> assuming all have components have to work in order for the system to work…

How to calc the failure rate in a system with identical, redundant components?

System reliablity = 1 - (1 - component reliability)^3

MTTF = integral from 0 to infinity over system reliability

system failure rate = 1 / MTTF system

What are additional requirements for HW/SW systems?

efficiency

cost, energy, execution time, weitht, area, …

often multi-dimensional optimization targets

real time capability

Last changed2 months ago