EECS 473Advanced Embedded Systems

Misc. thingsMidterm Review

Exam

• When and Where:– Tuesday 10/25 6-8pm– Rooms: 2505 GG Brown

• Coverage:– Everything we’ve done (lab, homework, lecture) other

than switching supplies • which we will restart/finish after break

• Study:– Sample questions in this presentation– Old exams– Labs, homework.

• HW1 answer key up later today (want to see common issues before posting)

Quick things before the review

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 status report

– Feel free to update the hard copy if you wish.

– We ask you questions– You ask us questions

• The goal here is not evaluation– It’s to help where we can

Group status reports

• Quick updates:– Solar car

– Laser pointer

– DriveRight

– FlexZone

– Headphones

• Next in-class report:– What are you blocking on?

– Where are your problems?

Also office hour changes for Seth And guest speakers

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 forboth 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 figureor 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(21/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 one of these over the others?

Group

T1

Execution

Time

T1

Period

T2

Execution

Time

T2

Period

T3

Execution

Time

T3

Period

CPU

Utilization?

RM

Schedulable?

EDF

Schedulable?

A 1 3 1 3 1 3

B 1 6 2 4 1 10

C 1 2 1 3 1 4

D 2 5 4 7

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.

1) Consider the following code for a Linux module. [12 points]

struct file_operations memory_fops =

{

.read = memory_read,

.write = memory_write,

.open = memory_open,

.release = memory_release

};

module_init (memory_init);

module_exit (memory_exit);

int memory_major = 60;

char *memory_buffer;

int

memory_init (void)

{

int result;

result = register_chrdev (memory_major, "memory", &memory_fops);

if (result < 0)

{

printk ("<1>memory: cannot obtain major number %d\n",

memory_major);} a) What is is “register_chrdev” doing? Describe how each of the 3 arguments is used. [6]

b) The module_init() and module_exit() do something very similar to the memory_fops

initialization code. Why are they being done in a different way? Address why we don’t use just

one scheme or the other. [6]

b) Describe the role of the memory_fops struct. In particular, what wouldhappen if we changed the .read line to “.read = bob”?

2) What is the difference between the “major number” and “minor number”?

3) How do you create a character device that is associated with a given LKM?

Licensing

• What is 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”

PCB questions (F12 midterm)

• Say you’ve done a PCB design at work. There is one power trace that is highly sensitive to additional resistance (it is the power line for an FPGA that has strict voltage requirements). Your boss says that “Your trace is too long. You’ll get a lower resistance if you neck down the trace and shorten it.” a) Sketch and label a diagram that illustrates the situation (including

why your trace was so long to begin with) and how your boss wants you to fix the problem

b) Say that the original trace was 5 cm long and 30mil wide and that after your follow her suggestion, the new trace is 2 cm long and 30 mil wide for all but 5 mm of those 2 cm where it is 6 mil wide. Would you expect the new or old trace would have a lower resistance? Show and explain your work.

c) Assuming her solution does reduce the resistance and still meets the PCB design rules; identify the biggest problem that should none-the-less concern you with this change. [5]

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– But, as mentioned in class, putting two battery packs in

parallel that aren’t balanced (same charge) for some reason can cause problems.• Where problems can include a fire.

– Generally, you should instead buy a battery with a higher capacity.

• Overcharge examples:• https://www.youtube.com/watch?v=gz3hCqjk4yc• https://www.youtube.com/watch?v=IsgnZCEeqsE

Other things

• Linear regulator

– Understand ideal

• Or nearly ideal with just a constant IQ.

– Be able to read and use a Linear regulator part specification

Example

• You have a linear regulator with a 6V input and 3V output.

– The output current is 100mA

– The quiescent current is 2mA

What is the % of power that is wasted by the linear regulator?

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.