Monday, September 29, 2014

Protip: max safe AVR clock speeds at 3.3 volts.

One thing I love about AVR chips is how electrically hardy they are compared to most ARM devices. Most can run at 5 volts, and source tens of milliamps. Most also run well at 3.3 volts, which is especially good when interfacing with an ARM.

One thing to watch out for is that the maximum stable clock speed for an AVR is reduced when running at lower voltages. ATtiny85s cannot run reliably at the internal pll'ed clock speed of 16MHz, when powered at 3.3 volts. I've cried myself to sleep over this, so I offer this cautionary tale. Read your datasheets!


Tuesday, September 23, 2014

Domain woes.

I'm having some issues with my new domain for this blog, so I've temporarily reverted it to the old domain. Atom feeds may or may not be working and please disregard any wierd redirect warnings ove rthe next few days while I work it out.


Working remotely on a Raspberry Pi

I've been trying to do some real work on a Raspberry Pi and it's cramped my style a little bit to cannibalize a monitor and slap an extra keyboard an mouse on my desk.

Working in Raspian Wheezy, you have a lot of Debian Linux at your disposal, so I thought, "I'll just ssh in" That works great, from either a Linux VM, or using Putty on Windows. I needed to run graphical applications and spawn terminals at will, so I really wanted full desktop experience.

X11, the base graphic interface run by virtually all Unix-like operating systems, supports a feature known as display redirection. I used this back in the 90's when I tried to make use of a boat anchor DECstation from my Slackware Linux box. It still pretty much works the same way it did 20 years ago. Before I get too far into how it works, I'm just going to stop and and mention that if all you want is to remotely run graphical programs on your Rasberry Pi, stop right there!  There is a much easier way! X11's server-client model is very powerful and flexible, and is unique in a class of technologies that has stood the test of time, but it's not very "get'er done" user friendly.

Linux machines support Microsoft's "Remote Desktop" protocol with two programs: xrdp and rdesktop.

xrdp is the "Remote Server". This runs on the machine that your want to remotely log into. Note that is this backward from X11, where the "server" is the machine with the physical display, and the client is the (remote) application that generates contents to display.

To install xrdp on your Pi (if using Raspian or other Debian Linux derivative), type:

sudo apt-get update


sudo apt-get install xrdp

You're pal apt will download, setup, and launch the xrdp deamon to start listing for connection requests. If you're parnoid, reboot your Pi to make sure.