Microchip Pic 18F2455/2550/4455/4550 Oscillator Settings Explained

I have a new fav Microprocessor. The Microchip Pic 18F2455. This is a feature-full yet powerful little device has almost limitless possibilities. Alas, you must understand that I am coming from the Pic 16F77, which lacks USB support, but is far more simple to get "blinky lights".

What the 18F2455 achieves in features, it gives up in ease of use (for newbies like myself). The external oscillator settings where an epic nightmare to get right! I believe the reason for this is twofold:

1. Nanowatt (tm) specifies the use of 2 clocks/oscillators for the 18F2455. The first clock for the main clock source to run the cpu itself; the ALU, the USB port(s) and all that. The second clock runs timers. This setup allows the main clock (and CPU) to power down leaving the second clock running to handle checking for new data on the USB port and running timers.


2. The USB port needs a strict 48 MHz clock to run, but Microchip still wanted to allow users of these chips to use oscillator configurations that they were used to. For example, for me, I have always clocked my chips at 20 MHz and I have a surplus of those crystals, so using a 20 MHz clock source would be really nice. I can still do this, but I need to use internal chip configurations to make use of the chip's internal pre/post oscillator scalers.

Here is how it all breaks down for me:

using 18F2455 and a 20 MHz external crystal. Please open the data sheet for the 2455 and refer to figure 2-1.

diagraming it out:

20 MHZ --> pins 9,10 (on pic)

you can see from Figure 2-1 that whatever you hook up to pins 9,10, you are feeding a PLL prescaler internally and you MUST feed the subsequent PLL prescaler a 4 MHz clock source as its input. This means that for me, I have to prescale my 20 MHz oscillator to 4 MHz. This means that we need to divide 20 MHz by 5. You can see from figure 2-1 that this bit setting is 100 on the PLL DIV MUX.

Now you have 96 MHz at the exit of the second PLL that is feeding another divider and also feeding a PLL postscaler. So then you can set the CPU DIV for the appropriate div for the main CPU clock source. This is 1 in my case because I want the main CPU to be clocked at 20 MHz.

If you are thinking that this is pretty confusing, you are right! There are a lot of choices (12 different oscillator modes) in oscillator setup and configuration. This leads to confusion and dismay. Mayhem might ensue. But, I think if you take the time to read Fig. 2-1 and learn the register bits that control which mode the clocks can be in, then I think this is an extremely powerful tool.

Montana Property

I wish I had the money and freedom to buy this property myself.

Check this beautiful Western USA property out!

52 acres for sale in Noxon Montana

SSH: setting up key authorization only (authorized_keys)

This is a great site for dealing ssh key authorization.

http://www.eng.cam.ac.uk/help/jpmg/ssh/authorized_keys_howto.html

The problem with sites that help you setup ssh key authorization is that they typically leave out the specific chmod commands (permissions) that you need to make the darn thing work!

What I mean to say is that people need to see an example of how this works and not just out of context commands and scripts.

Here is an example of a Mac OS X ssh client trying to setup ssh key authorization with a CentOS ssh server.

on the Mac, issue the following commands (you may or may not want a pass phrase on your private key), also substitute your username and hostname in where the double quotes are.

1. ssh-keygen -t dsa

2. scp .ssh/id_dsa.pub "user"@"CentOS host":~

3. ssh "user"@"CentOS host"

on the CentOS server

1. mkdir .ssh (this directory may or may not already be there; it should be safe to issue either way)

2. cat id_dsa.pub > .ssh/authorized_keys

3. chmod 755 .ssh && chmod 755 .ssh/authorized_keys

on Mac again: (hit ctrl-D to log off the server)

1. ssh -vvv "user"@"CentOS host"

The ssh client should log you in without a password if there is not password protected private key. If you protected your private key with a password, then it should ask for the password for the private key. Ssh should NOT ask you to provide a login for the shell on the remote side. The triple 'V' will display a verbose list of the conversation going on betwix client and server.

Modulo operators with respect to Ruby and Javascript

Recently, I was tasked with creating a calendar which transitioned from month to month such as December to January to February, etc.. Also, I needed to go backward on the transition.

A good way to do this is to use a modulo operator. This means that you can figure out which month is the next month number very easily.

So if the month is December, which is month number 12, to find out which month is next:

month = (month % 12) + 1 or (12 % 12 )+ 1, this will work for any month number from 1 to 12.

BUT to go backwards on the month numbers, here is the operation:

month = ((12+(month-2)) % 12) + 1 ... the funny thing is that this operation does not work in Ruby, but it does in Javascript.


In Ruby, here is the equivalent backward operation:

month = ((month-2) % 12) + 1


What is the difference? Why would there be a difference? I have no idea. It seems like Javascript would generate the right response to -1 % 12, which is -1 because 12 divided by -1 is 0 with -1 left over, but Ruby, Python, Perl, and the Google calculator figure that it should be 11 left over. This may be because a negative remainder would make no sense. So if the modulo is negative, then add it to the second operand to get the real modulo.

I must speak out on the this one

Recently, while at work, I was discussing why Google maps do not include a scale. In fairness to Google, the maps may have this functionality, but I am ignorant as to how to enable this feature.

My hypothesis as to why there is no scale was a simple one. To accurately determine scale for a map, one would need to consider the topography of land the map covers. Maybe Google just does not want to go to the trouble.

Seems simple enough to me, but to my surprise, the other two parties in the conversation said that topography has nothing to do with scale on this map. Huh?!? I stipulated the point to avoid a nasty argument where more than one person might need to get schooled, Sesame Street style.

Well, I can't take it anymore; time for some schooling. The assertion I am attempting to buttress is that topography (whether there are mountains and valleys on the surface of the earth) matters when applying a scale marker on a map. Google could probably estimate this very accurately since topography overlays are available to them, but who wants to do calculus for such a small thing? Anyway, here is an image which details my explanation.

Installing Rails 3 on Debian Unstable (Sid)

Recently, I was installing Rails3 on my Debian unstable machine and I noticed an error that was really strange.

Here was the error:

ERROR NameError: uninitialized constant ActiveResource::Base

The problem ended up being that I was missing this deb file:

libopenssl-ruby1.8

I have seen posts from people proclaiming that uninstalling old versions of activeresource would do the trick, but I reinstalled activeresource-2.3.5 gem after Rails 3 starting working again without a failure. I hate cryptic errors like that.

Mac OS X Leopard installing the ruby gem eventmachine

I was having some trouble installing eventmachine (a ruby gem that needs an extension library written in C++) recently in Mac OS X Leopard. I am currently running ruby 1.8.8dev. I think, perhaps, when I compiled ruby from source, the config left out the CXX flags setting. This makes mkmf not know what C++ compiler it needs to set in the Makefiles it creates. Since there are not too many gems that have extensions written in C++ (that I know of), I never really ran into this issue. Here is how to fix this.

Edit this file inside the directory of the gem, in my case;

$ vim /usr/local/lib/ruby/gem/1.8/gem/eventmachine-0.12.11/ext/extconf.rb

Right above the last line (the last line being a call to create_makefile), insert this line:

CONFIG['CXX'] = g++

then do the same thing for this extconf.rb in the fastfilereader dir (or anywhere else in the ext dir where there might be more nested extconf.rb files.

then run this:

cd /usr/local/lib/ruby/gem/1.8/gem/eventmachine-0.12.11; rake gem:install

this should solve the issue of getting errors like this:

make: I.: Command not found
make: dynamic: Command not found