As noted previously, I’m using MogileFS as the data store for some high(-ish)-performance computing. So far, it does the job without complaints. I haven’t benchmarked it, but I’m not asking that much.

One issue is that it uses its own proprietary access protocol … sortof. A simple Mogile read goes something like:

1. Computer A decides it wants a file from the Mogile cluster. It contacts one of the trackers using the Mogile protocol. [Computer A needs to know the hostnames of the tracker(s) in advance.]
2. The tracker has a think, checks the database, and returns the path to the requested file on one of the storage hosts. This path is a normal URL. It passes this URL back to the client.
3. Computer A uses HTTP to get the file from that URL on the storage host.

The fly in the ointment, of course is that the original client needs to speak Mogile to the tracker. I’m using a hacked version of SeattleRB’s Mogile-client Ruby gem in all of my various programs and clients to access Mogile directly.

One of those programs is a web front-end for browsing the data. When it wants to insert a Mogile asset into a web page (an image, typically), it will contact the tracker, get the storage URL, and insert it directly into the page, so you end up with HTML that contains code like:

``` <img src="http://my.mogile.storaged:7500/dev1/0/000/405/0000405859.fid">```

The problem is that all of my Mogile trackers and storage nodes are behind a firewall (as they should be). When I’m on campus, this works fine, as my desktop can see both the web server and the storage nodes.

When I work off campus, I can use an SSH tunnel to get to the web server, but all of the storage node paths break terribly.

I’d like to add a web proxy, preferably running on the same web server as my web app, which can handle the Mogile requests. Instead of returning raw Mogile URLs, the web app can pass paths to the proxy, the proxy can get the data from Mogile, and send it back as HTTP. As my web app is mostly for browsing, the proxy can even be read-only.

## How I did it

As a first attempt, I was more interested in getting a solution running than any sense of performance. As my web app is in Rails 3, I knew I could basically delegate a path to a separate, self-contained Rack server. The Resque control panel does this, for example.

As a sketch, then, I want any path of the form:

``` http://webserver.com/mogproxy/some/key/something ```

to return whatever Mogile has stored under the key `/some/key/something`

The first step was to add a route to the application in `routes.rb`:

I put my application code at `lib/mogproxy/` which resulted in a bit of a search on autoloading files and require paths. The solution I arrived at looked something like this in `application.rb`

It may be possible to do away with the `require` through clever use of the autoload path, but I didn’t feel the automatic-ness was really worth it. Note the require has to go at the end after the autoload_path is set.

The application below is written in Sinatra because I’ve used it before and it’s handy. It’s pretty simple — take whatever path comes in and look it up in Mogile. Return the result.

[Yes, I know, my regexp-fu is weak…]

As a side note, the `MogileModule::Mogilable` and `mog` bits are code I wrote to make a class (*cough* global) variable Mogile client instance, which can be configured once and used throughout the application.

It’s hardly efficient because it loads the asset from Mogile then spits it back out at the client. And the error checking is basically absent. But it works.

On the other side, I then rewrote my client code to return “/mogproxy/….” paths instead of looking up the mog paths themselves.

## A small MSP430 board

I couple of months back I acquired a TI Launchpad, their loss-leader development board for their MSP430 family of microprocessors. I’ve had a long-term interest in the MSP430 as an alternative in the realm of 8-/16- bit micros.

Out of the box, I was able to the set up the mspgcc toolchain, and the other installation details were amalgamated from the interwebs. I believe the msp toolchain is now in the Ubuntu repositories, so all of the compilation la-di-da is a thing of the past.

Sadly, I was without an application.

The next week my coworker asked me where he could find a signal generator — I made the mistake of inquiring and it turned out he needed a 30Hz (later 25Hz) synchronization pulse for a cluster of computer vision cameras. Clearly a signal gen would be massive overkill (and would tie up the generator for months to come), so I offered to make a signal generator for him.

My first version was built on the Launchpad, and worked fine. Last week I finally got around to constructing a self-contained version.

Total time to first prototype was perhaps four hours, though I went through three iterations of the clocks — first using the internal very-low power oscillator (VLO), then the internal digitally-controlled oscillator (DCO), and finally using an external 32.768 kHz watch crystal. Each step was an attempt to find a little bit more timing accuracy.

For the curious, the schematics are here

It’s not the minimum number of components to run an MSP430, but close enough. Nor is it the smallest MSP that would do that job. But the G2211 came with the Launchpad.

The USB port only provides power (it’s convenient and there’s no chance of getting the voltage or polarity wrong). The C connecting reset to ground is recommended, but I’m not sure it’s mandatory. It should provide a little highpass filtering on the reset line. As built there’s no in-circuit programming, hence the socketed processor. It wouldn’t be too hard to add an ISP but I didn’t see the point.

The code is as follows (or here)

```/******************************************************************************
* 25Hz flasher.  Ripped unashamedly from the Launchpad demo app.
* This version is targeted at the 430G2211 with a watch crystal
******************************************************************************/

//#include   <-taken care of by includeing io.h and setting -mmcu=msp430x2012 in cflags
#include
#include

#define     LED0                  BIT0
#define     LED1                  BIT6
#define     LED_DIR               P1DIR
#define     LED_OUT               P1OUT

// The Launchpad has two LEDs
// but the app board has just one
#undef      TWO_LEDS

#define     TA_OUT0               BIT2
#define     TA_DIR                P1DIR

void main(void)
{
WDTCTL = WDTPW + WDTHOLD;                 // Stop WDT

TA_DIR |= TA_OUT0;
P1SEL  |= TA_OUT0;                        // Should set Pin 1.2 to TimerA out1

#ifdef TWO_LEDS
LED_DIR |= LED0 + LED1;
LED_OUT &= ~LED1;
#else
LED_DIR |= LED0;
#endif
LED_OUT |= LED0;                          // To enable the LED toggling effect
// Use a 32.768 KHz watch crystal
BCSCTL1 = XT2OFF;
BCSCTL2 = SELM_3 | SELS;                   // MCLK = SMCLK = LFXT1CLK
BCSCTL3 &= ~(LFXT1S_3);                    // Clear LFXT1Sx for 32678 crystal on LFXT1
BCSCTL3 |= XCAP_3;                      // Set ~12 pF capacitance

// As set up, MC_1 means count to CCR0
// OUTMOD_7 means reset at CCR1, set at CCR0
// ISR make the LEDs behave similarly -- RED on at CCR1, GREEN on at CCR0
//
// Uses toggle mode, OUT1 toggles every time TimerA == TACCR1
// And TimerA is in Up mode, so it counts 0->TACCR0, then resets
// So the half-period of the output is set by TACCR0
//
// 32768 / 655 = 50.02
//
// So ~655 for 25MHz
TACCR0 = 655;                             // By calibration
TACTL = TASSEL_1 |  MC_1;                  // TACLK = SMCLK, Up Mode
TACCTL0 = CCIE;
TACCTL1 = OUTMOD_4;                // Toggle mode
TACCR1 = 0;

WRITE_SR(GIE);                     // Enable interrupts

/* Main Application Loop */
while(1) {
__bis_SR_register(LPM3_bits + GIE);          // LPM0 with interrupts enabled
//nop();
}

}

interrupt(TIMERA0_VECTOR) ta0_isr(void)
{
static unsigned int count = 0;

TACCTL0 &= ~CCIFG;

if( ++count >= 25 ) {
#ifdef TWO_LEDS
LED_OUT ^= (LED0 | LED1);
#else
LED_OUT ^= LED0;
#endif
count = 0;
}

}
```

Not the tidiest code in the world. The 25Hz square wave is driven by the TimerA, while the flashing LEDs (the Launchpad board has two blinkenlights, while my board has just one) are driven my a more conventional counter-in-an-ISR. I’m quite sure I could fix it up so the LED is controlled by a timer as well, but I haven’t bothered.

## Resistor search in Ruby

Here’s a short snippet I wrote the other day. I think it’s a good illustration of Ruby’s potential for short scripts.

I needed to set the resistor divider to get 5.4V out of an adjustable LDO regulator (a REG102-A). The output voltage is set as:

$V_{out} = left( 1 + frac{R1}{R2}right) V_{ref}$

For the REG102, $V_{ref} = 1.26V$, and the bias current was about right if R1 and R2 were both in the order of 1-10K.

On the other hand, I had a big bag of completely unsorted SMD resistors. Somewhere in there was a nice combination that would give me the voltage I wanted.

The script reads a list of resistor values (one per line) from a file (by default ‘r.txt’). The output looks like:

```Have these resistors: 0.91, 2.2, 4.7, 6.81, 8.06, 10.0, 11.5, 15.0, 22.0, 24.9, 30.0, 33.0, 35.7, 40.2, 42.2, 47.0, 51.0, 53.6, 56.0, 68.0, 100.0, 215.0, 226.0
Searching for Vtarget = 5.4
R1	R2	vout	error
24.90	8.06	5.15	-0.05
68.00	22.00	5.15	-0.05
6.81	2.20	5.16	-0.04
35.70	11.50	5.17	-0.04
47.00	15.00	5.21	-0.04
215.00	68.00	5.24	-0.03
15.00	4.70	5.28	-0.02
22.00	6.81	5.33	-0.01
33.00	10.00	5.42	+0.00
226.00	68.00	5.45	+0.01
100.00	30.00	5.46	+0.01
51.00	15.00	5.54	+0.03
40.20	11.50	5.66	+0.05
```

The code loads the resistors from the file, and removes duplicates, then calculates Vout for every permutation. It then deletes any permutations which don’t get within 5% of the desired voltage, and sorts and prints the results.

As a side note, it doesn’t matter than the resistors are given in Kohms (or Ohms, or MOhms, etc) because they’re divided in the script, as long as they’re in the same units.

Yes, it’s non-optimized because it does build an enormous list of possibilities, then throws the vast majority away. Even a simple permutation like not calculating Vout if R2 > R1 would eliminate many possible pairings.

But, there you go.

```#!/usr/bin/env ruby

TARGETV = 5.4

filename = ARGV.pop || "r.txt"
r = (\$File.open( filename ) { |f| f.readlines.map { |l| l.to_f } }).sort.uniq

puts "Have these resistors: " + r.join(', ')
puts "Searching for Vtarget = #{TARGETV}"

vals =  r.permutation(2).map { |res|
vout = 1.26 * (1 + res[0]/res[1])
error = (vout-TARGETV)/TARGETV
res+[vout, error]
}.delete_if { |res| (res[3]).abs > 0.05
}.sort { |a,b| a[2] <=> b[2] }

puts %w( R1 R2 vout error ).join("t")
vals.each { |v|
puts "%.2ft%.2ft%.2ft%+.2f" % v
}
```