Not too long ago, we had #BlackLivesMatter protests that blocked some of the roads in the Boston area. There was much handwringing about how ambulances and other emergency vehicles were (potentially) delayed, but in that one-time event only one ambulance was diverted, and I heard of no particular harm from this one event.

Meanwhile, almost every work day there are traffic jams that impede ambulances. On the days that I shop at Fresh Pond Mall, in the few minutes that I am outdoors I often notice an ambulance slowed or even stopped by traffic.  I assume if I spent an hour watching during the rush that I would see one or more of these delays every single day.  The delays at the protests were larger, but if you roll the dice with small delays again and again and again, eventually there will be losses. Oddly enough, nobody makes too much of a fuss about these delays.

Notes on I2C on Atmega328p

February 7, 2015


TWBR = bit rate

  TWINT interrupt flag
  TWEA enable acknowledge
  TWSTA set start
  TWSTO set stop
  TWWC collision detected
  TWEN twi enable
  – reserved
  TWIE interrupt enable

  TWS(7:3) status
  – reserved
  TWPS (1:0) prescaler

TWDR = data register

SCL freq = CPU clock / (16 + 2 * TWBR * 4TWPS).

TWBR must be at least 10 for master mode.

SCL freq must be 100Khz or less.
CPU clock is 8 Mhz.
Denominator must be larger than 80 = 16 + 64, TWBR * 4TWPS must be greater than or equal to 32.
TWBR=10-31, TWPS=1 (40-124)
TWBR=32-255, TWPS=0 (32-255)
At 100kHz, send data at about 10kB/s. 80 characters requires 8ms.

Master-side state changes for writing a data byte to a slave:

Receive interrupt, check TWSR for successful start.

TWDR = SLA+W (W=0, R=1: SLA = 0x50 for Newhaven serial display
Receive interrupt, check TWSR for successful address.

TWDR = data
Receive interrupt, check TWSR for successful data.

repeat data.

Receive interrupt, check TWSR for successful stop. Read the rest of this entry »

Today’s Bad Driving

January 15, 2015

First, we start with someone who is doing something that requires two hands on the smartphone, surely not texting, that would be illegal:

Notice how the Prudent Cyclist is none too eager to pass this person after noticing their unpredictable and erratic driving.

Next, we have someone piloting their barge down the Broadway Ship Channel, and they have inadvertently strayed outside the marked boundaries.  Remember boaters, “Red, Right, Return”.

With studded tires, there’s not much to it.  If it’s flat, you go:

Not obvious in this video is that I was getting plenty of rear wheel spin after I passed the pedestrian.  I was just in a mood to scratch the ice, so I did.  It’s not at all clear that I needed the studs here, though the pedestrian seemed to think it was slippery.

If it’s a hill, sometimes you can’t go.  The rear wheel just spins, so you stop.  Careful putting your feet down, too.  It’s important to put the better tire on the front, so that “can’t go” is what happens:

Apologies for the heavy breathing; I was in a hurry to get home in the first, and climbing a 10% grade in the second.

Car Tire Noise

January 10, 2015

It’s a little thing, but sometimes people wonder whether electric cars are more of a hazard to bicycles because they are so quiet.  At low speeds, yes they are quiet, but at any reasonable speed (i.e., what you might expect to encounter outside of a traffic jam) their tires are plenty noisy.  Here’s a sample (taken on a windy day, too — on the other hand wet roads are a little noisier):

Yes, one of those cars is electric.

Bad noob experience with AWS

December 14, 2014

“Your recent Trac powered by Bitnami launch failed. Your requested instance type (m1.small) is not supported in your requested Availability Zone (us-east-1e). Please retry your request by not specifying an Availability Zone or choosing us-east-1b, us-east-1a, us-east-1c.”

And how, pray tell, is “retry my request” accomplished? I probed various links, none of them took me to the place where I launched from, do I need to delete this one first, or is it already dead? If I make the obvious mistake here, will it cost me money? (I don’t think it will, but there’s a gap between “don’t think it will” and “won’t”.)

And why did it let me make this request in the first place, if it was doomed to failure (I think this was all specified on an early page, why was this combo shown to me, never mind that it was chosen as the default)? And why isn’t there a help/feedback button on this page where I need help or want to give feedback?

Fortunately, I have a blog. Always remember, this is not just about public shaming of (other) bozos, this is doing them a favor by pointing out the upside potential in their products. People who care about improving their software will make it as easy as possible to file bugs, and will make it as easy as possible for the filed bugs to be informative to whoever has to deal with them.

I got a GoPro as a treat/present. Other people seem to use theirs to show how dangerous drivers can be (especially in London, what is it with London?), I figured it would be more constructive to show how things can work. And yes, sometimes drivers can be clueless and/or dangerous, that’s just the way our world works right now, but a lot of that risk can be managed.

Here are little bits and pieces of my commute, showing how various bits of safety advice play out in the real world. This is “non-legal” because so often the safety advice to cyclists starts and mostly ends with “obey traffic laws” as if that were either necessary or sufficient (and as if that were actually standard practice for drivers). The laws that people tell you to obey were not designed with bicyle safety in mind — sometimes they help, sometimes they don’t. They’re definitely not enough. The examples below illustrate rules I actually use.

My background is “long-term recovering Effective Cyclist” — I learned all the moves for riding in traffic, and I’m relatively comfortable doing that, but I think that overall that’s not going to work for most people. If it weren’t so necessary to “share” the road with drivers so often, this advice would be much less useful — but we’re stuck with crappy shared roads, so maybe this will be helpful to you. I recorded several commuting videos without specifically intending to demonstrate anything and then reviewed them looking for examples, so this is more or less rules-as-practiced, warts and all (I think I ride too close to the door zone, at least it sure looks like it on the video).

Without further explanation: Read the rest of this entry »


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