Raspberry-Python-Pi for Hardware Hacking Fun!

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Raspberry-Python-Pi for Hardware Hacking Fun! Daniel Bader – @dbader_org

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Hardware-hacking for software people.

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piradio

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Hi!

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• My project and the lessons learned • Python on the Raspberry Pi • Inspire you to try this too

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Why?

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How?

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Raspberry Pi

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Python on the Pi • Python is a ﬁrst-class citizen • CPython & PyPy (ARM) • A great platform for Python on an embedded system

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RaspiLCD kit by Martin Stepphun (emsystech.de)

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Soldering

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The case

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Custom CNCed front plate schaeffer-ag.de/en

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“Front-plate Designer” schaeffer-ag.de/en

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Margins

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Aluminum case Proma 1030 - conrad.de

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Parts list • Raspberry Pi Model B ($35) • Display + buttons (RaspiLCD) ($55) • Aluminum case + custom CNCed front-plate ($70) • 32 GB SD Card ($20) • Wi-Fi USB stick ($20) • USB power supply ($10) • Total: ~ $200 (in theory)

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Software

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Architecture

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Philosophy: Do as much as possible in pure Python.

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Architecture Linux piradio CPython mpd freetype raspilcd

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piradio services audio podcast clock weather … panels radio clock alarm weather … podcast Architecture graphics lcd fonts commons conﬁg ui

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Things I learned

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Simulate where possible

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• Implemented as a “fake LCD” driver • pygame + libSDL • Saved me so much time

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Take advantage of the familiar programming environment

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Fast Python (on the Raspberry Pi)

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1. Avoid the dots ! ! def bad(): a = A() ! for i in xrange(10000): a.b.c.d.e.foo() ! ! def good(): a = A() foo = a.b.c.d.e.foo for i in xrange(10000): foo() ! ! # 3.3x faster!

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1. Avoid the dots (...) ! FOR_ITER STORE_FAST ! LOAD_FAST LOAD_ATTR LOAD_ATTR LOAD_ATTR LOAD_ATTR LOAD_ATTR CALL_FUNCTION ! POP_TOP JUMP_ABSOLUTE POP_BLOCK ! (...) (...) ! FOR_ITER STORE_FAST ! LOAD_FAST CALL_FUNCTION ! POP_TOP JUMP_ABSOLUTE POP_BLOCK ! (...)

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2. range() vs xrange() range(): • returns a list • needs more memory • support slices • usually slower xrange(): • returns a generator • needs less memory • no slices • usually faster

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3. Access bitmaps row-wise instead of column-wise … = reality “memory layout” abstraction “2d bitmap”

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3. Access bitmaps row-wise instead of column-wise … = nice, sequential memory access

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3. Access bitmaps row-wise instead of column-wise … = jumping all over the place

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3. Access bitmaps row-wise instead of column-wise ! ! def bad(): bitmap = bytearray(w * h) v = 0 for x in xrange(w): for y in xrange(h): v += bitmap[y * w + x] ! ! def good(): bitmap = bytearray(w * h) v = 0 for y in xrange(h): for x in xrange(w): v += bitmap[y * w + x] ! ! # 1.1x faster (Pi)

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Fast Python (on the RPi) • Proﬁle and benchmark everything • Trade memory for speed: cache things • Use the right data structures

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Fun parts • Building a physical object • Building something useful and seeing people use it • UI design and programming in a restricted environment • Programming nostalgia – Pixels!

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Not-so-fun parts • Soldering • Front plate margins • Raspberry Pi audio crackling • Things get expensive quickly

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The Future • I'll release this as open-source on GitHub • Finish the blog post on it • Users and contributors welcome

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github.com/dbader/piradio dbader.org/blog/tags/python ! @dbader_org Thanks! Questions? Ideas?