Jeff Epler's blog

Fun with frequency

10MHz OCXO (top), 10MHz GPS-referenced (bottom)

• A new 4-channel scope
• A GPS-referenced tunable frequency generator
• A "PLJ-8LED-C" frequency counter from ebay
• An "AD9850 DDS freqency synthesizer breakout" from ebay
• A 10MHz OCXO from ebay years ago and miraculously rediscovered midway down a junk pile this weekend

Nothing earth shattering or super fancy, but it is still pretty amazing to be able to heat or cool the AD9850's (non-compensated) XO and see the digital frequency meter change. And it was gratifying to find that the OCXO can be tuned quite easily to 10,000,000MHz.

The PLJ frequency counter is adequate to its task. Supposedly it has a VC-TCXO, and while it has just a single turn adjustment for frequency, I was able to get it to consistently read 10,000,000MHz from OCXO and from GPS; the next day, it had drifted down to 9,999,999. Still, that's down in the .1ppm and is probably to be expexcted.

I found that when tuning the OCXO, it was much more useful to use the scope to visualize the changing relative phases of the two signals, than to use this particular frequency meter which needs a 1s gate time to measure a 10MHz signal at a precision of 1Hz, and can't do anything more precise than that.

To tune the OCXO, I set the GPS-referenced RF generator to 10MHz and hooked both up to the scope. With a time base of 100ns and a persistence of 10s, it's not only easy to fine tune the circuit, but a hand-wave estimate of the error by looking at the persisted signal. If it's 1 div wide, then the relative error between the two frequencies is 100ns/10s = 10ppb = 10 milliHz @ 10MHz.

The scope has a 1kHz signal for probe compensation and it also includes the signal generator option. To my surprise, the frequency error of the probe compensation was MUCH smaller than the frequency error of the signal generator!

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All integers 0 through 1,000,000 from short, digitless dc programs

I consider the 23 characters " ~^-/*+|ABCDdEFIinOorvz", but prune all prefixes which are invalid due to stack underflow. 23¹² is around 2.2×10¹⁶ or 2⁵⁴, but best guess my various programs have considered only around 5×10¹² or 2⁴² due to this pruning. Still, the process (including iteratively developing the search program) has taken several months!

What's next? I guess there's always the "fours puzzle", dc edition.

      4 4n
     10 In
      0 zn
     44 44n
      2 4vn
      1 4zn
      3 Ivn
    444 444n
      6 44vn
     40 4I*n
...

Files currently attached to this page:

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Code size for bit reversing algorithms

Code space is always at a premium, so I investigated several code fragments for bit reversal to find out which was smallest on arm and xtensa. These code fragments are gathered from the internet. The loop is the smallest alternative on both architectures, but the 16-element look up table is not much bigger (on arm, the difference is bigger on xtensa) and is probably faster.

arm-none-eabi-gcc-7.2.1 -Os -mthumb

   text    data     bss     dec     hex filename
     40       0       0      40      28 bitrev_loop.o
     44       0       0      44      2c bitrev_lut16.o
     44       0       0      44      2c bitrev_shifts1.o
     44       0       0      44      2c bitrev_twiddle1.o

xtensa-lx106-elf-gcc-4.8.5 -Os

   text    data     bss     dec     hex filename
     40       0       0      40      28 bitrev_loop.o
     53       0       0      53      35 bitrev_lut16.o
     57       0       0      57      39 bitrev_shifts1.o
     52       0       0      52      34 bitrev_twiddle1.o

Files currently attached to this page:

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Fuzz-testing CircuitPython

(M)(L)

afl-graphs

I recommend that you use a throwaway virtual machine for this, because at one point afl-fuzz learned how to create files in the filesystem! that was a big surprise, waking up to a directory full of filenames like "tesppppppppppppppppppppppppppppptfile"!

First, make sure you can build circuitpython's unix port. The steps are, approximately,

1. Clone circuitpython
2. git submodule update --init --recursive
3. make -C ports/unix -j5 deplibs
4. make -C ports/unix -j5

Note that the executable is ports/unix/micropython even when you have cloned circuitpython.

Next, get afl from http://lcamtuf.coredump.cx/afl/. If you can, follow the instructions in llvm_mode/README.llvm to get afl-clang-fast. Now, clean and rebuild:

1. make -C ports/unix clean
2. make -C ports/unix CC=/path/to/afl-clang-fast -j5 deplibs
3. make -C ports/unix CC=/path/to/afl-clang-fast -j5

Prepare the testcases directory for afl-fuzz. I used a number of tests from tests/basic:

1. mkdir testcases
2. cp tests/basics/*.py testcases

And start the fuzzer:

1. /path/to/afl-fuzz -i testcases -o findings -- ports/unix/circuitpython

If you have any good findings, drop by the adafruit circuitpython discord and let us know about them! Even better if you fix them.

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Watch Repairs

(M)(L)

My Grandfather's Hamilton Railroad Watch, repaired by Chris Radek

After a 15-year break, I'm now accepting repair work again!

I do everything from routine cleaning and oiling (full disassembly and inspection, multi-stage ultrasonic cleaning, reassembly and correct oiling using appropriate modern synthetic oils, timing and adjustment as necessary) of automatic and manual-wind wrist and pocket watches to manufacture of unavailable parts for chronographs or repeaters.

I specialize in Accutron tuning fork watches and I have a large stock of parts and many years of experience working on them.

Please contact me and tell me how I can help you. If you are a collector I've previously worked for, I'd especially like to hear from you.

His work winding new coils for Accutron watches is particularly exciting; he tells me he's heard of just one other person doing this in the last 20 years. These particular watch parts are otherwise irreplacable.

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Finding all numbers produced by short, digitless 'GNU dc' programs

A few years ago, possibly in 2014, I saw this code golf problem, which invites you write a short program that prints "2014"—the caveat being, the program may not contain any digits in its source code.

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Four years of Prius (2013) fuel efficiency

Fuel economy and total mileage both went up in 2017 compared to earlier years. However, due to taking more road trips total fuel usage went up.

This year had my best mileage on a single nontrivial trip, shown as 99.9 MPG on 58.5 miles. This drive included significant downhill portions (descending from 12,000 feet to 6000 feet, if I recall correctly).

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poc: Python OCE Composer

Here you can see that the input format is fairly terse, and it has some abilities OpenSCAD doesn't, like selectively filleting certain edges:

with Difference():
    Box((-50,-50,-50), (50,50,50))
    Cylinder((-100,0,0), (100,0,0), 25)
    Cylinder((0,-100,0), (0,100,0), 25)
    Cylinder((0,0,-100), (0,0,100), 25)
Fillet(12, [e for e in Edges() if e.boundingBox().min.z > 0])

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occmodel as Python alternative to openscad

MP Select Mini: Heated Bed Repair

My internet usage would fit on six 33.6kb/s modems on average

Monoprice Select 3D Printer Review (part 4)

Monoprice Select 3D Printer Review (part 3)

Time Reversed Cosmology

Monoprice Select 3D Printer Review (part 2)

Three years of Prius (2013) fuel efficiency