EECS 473 Advanced Embedded Systems Misc. things Midterm Review

Exam When and Where: – Tuesday 10/27 6-8pm – Rooms: 1500 EECS & 1003 EECS If you have a conflict, let Yitian and I know ASAP. Coverage: – Everything we’ve done (lab, homework, lecture) other than switching supplies. Study: – Sample questions in this presentation – Old exams – Labs, homework. HW1 answer key up on Friday

Milestone meetings Tuesday during the day – No class on Tuesday – Doodle out later today. Basically – You tell us where you are at bring 3 copies of MS1 – We ask you questions – You ask us questions The goal here is not evaluation – It’s to help where we can

Start the Review Interface design for hardware Real-time systems – Scheduling Licensing issues Software platforms – Barebones – Barebones w libraries (Arduino) – RTOS (FreeRTOS) – Full OS (embedded Linux) PCBs and power – Terminology – Power integrity – Batteries – Linear regulators including LDOs Design – Expect a design question

Topic: Interfacing Writing software interfaces for hardware – Ideally have a standard interface for both hardware and programmer. Makes it easy to port software. Also means it’s obvious what hardware control to provide. – Like any interface, standardization here is very powerful, but comes at a cost. Abstracting away interface issues makes things less efficient. » Examples?

Interface questions Might be asked to design an API Might be asked to critique an API – Arduino, Tinkerforge worth looking at. Might be asked to explain this figure or ideas related to it. Might be asked to explain or apply this figure:

Topic: Real-time systems and scheduling Time matters – Hard, soft, firm deadlines Validation if very difficult – How do you know the worst case timing? Really difficult to prove worst case. Cache misses, branch prediction, etc. make for a very complex situation. For safety critical things, even a “large engineering margin” isn’t enough. – Need to actually figure it out. "those systems in which the correctness of the system depends not only on the logical result of the computation, but also on the time at which the results are produced";

Topic: Real-time systems and scheduling Rate monotonic scheduling – Static priority scheme – Assumes all * tasks are periodic. Give priority to tasks with lower period. – Total utilization helps figure if schedulable. If is less than n(2 1/n -1) (n=number of tasks) it is schedulable. If over 100% not schedulable If neither is true, do critical instant analysis. EDF – Requires dynamic priorities – Works if less than 100% utilization

Example scheduling questions Find a set of tasks with % utilization, period, etc. that RM can’t schedule but EDF can. Solve specific problems: Explain properties of EDF/RMS/RR/FIFO. – We didn’t do much in class with RR and FIFO… Why might we prefer RMS over EDF? Group T1 Execution Time T1 Period T2 Execution Time T2 Period T3 Execution Time T3 Period CPU Utilization? RM Schedulable? EDF Schedulable? A B C D 2547

Topic: Software platform We covered three or four basic platforms for software development for an embedded system. – Barebones Write everything yourself – Barebones plus libraries Import some useful libraries but otherwise write it all yourself. – RTOS Basic scheduler with a lot of control Generally a fair bit of support. – I/O devices, memory management, etc. Fast interrupts processing possible/reasonable/”easy” – Full OS Give up a lot of control Have to deal with a very complex system Get lots (and lots) of software support – Vision, databases, etc.

FreeRTOS Tasks and scheduling – Creating tasks (xTaskCreate) – Semaphores – Deferred interrupt processing. – Can dynamically change priority.

FreeRTOS questions Given needed function prototypes used in lab: – Write code which does RM scheduling of two tasks. “A” has a period of 10 clocks and runs for at most 4 clocks. “B” has a period of 6 clocks and runs for at most 3 clocks. – Might also be asked to handle deferred interrupts and semaphores What is the difference between Ready, Running, Blocked and Suspended in the diagram to the right?

Embedded Linux What limitations on real-time you might have Can be fairly small – Things like busybox help I/O has a standard interface – File model Not always ideal. But there is a lot of complexity here – We spent a fair bit of time writing drivers.

More questions What is busybox? Why is it useful? Explain how a “file” in /dev gets linked to a given LKM.

Licensing What a viral license is – Why it matters in embedded perhaps more than elsewhere. LKM Impact on business model Hardware people tend to use a lot of other people’s code (legally). – Vendor’s driver code etc. – Libraries. GPL and Creative Commons Licensing.

PCBs Basic terminology and issues – Trace, mil, thou, via, silkscreen, clearance, layers, rat’s nest, etc. – Through-hole vs surface mount – Schematic vs. Layout – When to “neckdown”

Power integrity Discuss keeping Vcc/GND constant as possible. – Recognize that our devices can generate current draw variations at a huge number of frequencies. – Spikes or droops could break our device. Need caps. – Small and large – Get right values

Batteries Understand mAh – Understand that mAh will be less if draw too quickly. – Be able to work basic math using specific battery properties.

Simple battery example 800mAh battery. – If we need 3.5V (or more) how long will this battery last at a 1.6A draw?

Battery note Be very careful putting batteries in parallel. – Homework question made it seem easy And it generally is – But putting two battery packs in parallel that aren’t balanced (same charge) for some reason can cause problems. Where problems can include a fire. Overcharge examples:

Other things Linear regulator – Understand ideal Or nearly ideal with just a constant I Q. – Be able to read and use a Linear regulator part specification

Design Expect a design question – We’ll give you the datasheet (or needed parts of a datasheet) for some part. – You’ll be asked to do some design. – Code will probably be Arduino (easiest to work with) We’ll also give you any Arduino APIs needed.