Month: April 2016

PDB for n00bs

PDB is the python debugger, which is very handy for debugging scripts. I use it two ways.

If I’m having a problem with the script, I’ll put in the line

import pdb; pdb.set_trace()

just before where the problem occurs. Once the pdb line is hit, I get the interactive debugger and can start stepping through the program and seeing where it blows up, and what variables are getting set to before that happens.

However, I recently found a very handy second way. I was debugging a script with a curses interface, which cleans up when it exits. Unfortunately, that cleanup means that my terminal gets wiped when something crashes, so instead of a stack trace, I just get dumped back to the terminal when something goes wrong, with no information at all left on the screen.

Invoking the script with

python -m pdb ./

gets me the postmortem debugger, so when something goes wrong, the program halts and I get the interactive debugger and some amount of stack trace. It’s messy looking because of curses, but I can at least see what is going on.

New Swarm Controllers

I’ve ordered the second version of the swarm control boards. If you want some, you can get them here, but I advise against doing so until after a post shows up here saying either that they work, or that they’re busted.

In the mean time, I’ve been realizing that the boards are good for all sorts of stupid tricks. For instance, you can control people using galvanic vestibular stimulation, which uses 1-1.5mA at pretty low voltages (More academic version, more hacking). Since the swarm control boards already use a 3.7v lithium cell, additional voltage regulation isn’t needed (if anything, they may be too weak), and PWM can be used to control the current. A resistor in series might also be good, in case of… errors.

The same board could also be connected to a door latch, or magnetic strike, which would let a user connect to a web page (the ESP8266 can serve web pages and act as an AP) and put in a password to open the door. Lockitron appears to be making a business out of selling this, but the mechanics are cheaper.

Given that there’s also an I2C bus on the device, IO expanders, sensors, and other goofiness could be added to make wearables that respond to the environment, smart dust sensors, IoT nodes for home automation, scales that tweet about how much you weigh, etc. IoT is the new black! It’s a floor wax! It’s a dessert topping!




ESP8266, Serial Adapters, and Resets

As detailed in the previous post, I’ve been having some trouble getting the Arduino development environment to automatically reset my ESP8266 board using the DTR and CTS lines of the serial adapter. Part of my problem may still have been the cheap serial adapter, but today I found a new part.

The ESP8266 is extremely sensitive to noise on the CH_PD line, and I was using a 9″ long jumper to connect CH_PD to RTS. I confirmed with my O-scope that RTS was pulsing as it should, but the first pulse threw the ESP8266 into some weird state where it spewed noise on a bunch of pins (GPIO0 seemed to be the worst), and uploading would, naturally, fail.

Switching to 3″ jumpers cleared up the problem and let my Arduino IDE reset the ESP8266 as it should.

I’ve changed the schematic in the Github repo for the project to reflect the new reset wiring, but I still have to add a 5V input connection for charging the battery. Once that’s done, I can design a new PCB.


A Word of Warning

My PhD work (TinyRobo) uses a USB-Serial converter to talk to the ESP-8266 modules in the tiny robots. Normal FTDI cables have a cable that ends in a 0.1″ 6-pin header with this pinout:

  1. Black – Ground
  2. Brown – CTS
  3. Red – VCC
  4. Orange – TX
  5. Yellow – RX
  6. Green – RTS

It turns out that esptool can manipulate the DTR and RTS lines to reset the chip in bootloader mode, which is great for uploading code to it. It also means I can get away with not having any parts on the TinyRobo boards to handle the reset, which is great because it lets me keep the board small. Unfortunately, the FTDI cable I have doesn’t expose the RTS line, so I got a converter module for cheap off Amazon. The particular module I got is this one:

2016-04-15 14.10.13

I added that red wire and cut a trace so that the pins would be:

  1. DTR
  2. RX
  3. TX
  4. VCC
  5. CTS
  6. Ground

So far, so good, but I can’t upload with it. I threw a scope on the lines, and it looks like instead of swinging from VCC to ground like well-behaved TTL serial lines, they swing from VCC to VCC minus some tiny voltage, less than a volt. Adding pull-downs on the lines doesn’t seem to have helped. It could be that the timing is of, but I suspect that somewhere, some cheapskate saved some fraction of a cent on this board, at the expense of it doing the one thing it was supposed to do (YOU HAD ONE JOB).