Wednesday, December 7, 2011

The Loginator

Howbout the anti-gravity evil launchinator. There, I changed the name, that makes it mine.
--Dr. Heinz Doofenshmirtz

So I took advantage of the Open Hardware nature of the Sparkfun Logomatic, made a few tweaks and ended up with the Loginator. This board is 100% backwards pin compatible with the Logomatic and 99% firmware compatible. It has a number of cool features and annoyance fixes, described below.

New features and improvements:
  • Boot button - This shorts the USB detect line, causing the USB bootloader to think that the USB has become disconnected. This causes the bootloader to check for a new main firmware, install it if necessary, then run it. Basically it prevents having to unplug and replug the USB when you are debugging things and want to rapidly start the main firmware. Plus, this way you don't need a battery to maintain power across the USB unplug, so you can run the main firmware without a battery.
  • RGB LED - This replaces the STAT lights on the Logomatic. The red and green lights are connected to different pins on the microcontroller, so the Sparkfun firmware will have to be slightly modified to accommodate. The blue light is available also.  Formerly the lights were attached to the same pins needed for I2C. Now they are attached to PWM pins. Unfortunately, two of these PWM pins are connected to TX and RX for UART0. There is a solder bridge next to the LED. When shorted, the lights work, and when not, the lights are out and don't interfere with the UART. If you leave them on and use UART0, then the lights will blink as the port transmits and receives.
  • I2C ports - The LPC2148 has two hardware I2C ports. Unfortunately on the Logomatic, these pins are used for things like the STAT lights and STOP button. The Sparkfun firmware will have to be slightly modified to use the new stop button pin.
  • Current sensor - The Logomatic had a battery voltage sensor set up on an otherwise unused ADC channel. The Loginator includes this, but also a current sense resistor and amplifier on the input to the 3.3V regulator so you can see how much current the device is using. This current sensor uses a pin which was not connected on the Logomatic.
  • Improved power supply - The Logomatic could run on USB only, and could run on USB while simultaneously charging the battery, but only do to a quirk in the particular charging circuit used in the Logomatic 2.3.With no battery connected, the charging circuit powers the load only because it thinks that the rest of the Logomatic is a battery to be charged. I tried a couple of newer non-obsolete chargers and couldn't get them to work like this. The new circuit has a set of bypass diodes such that when the USB is plugged in, it explicitly powers the circuit and cuts the charger off from the rest of the circuit. On battery power, the diodes cut off the USB, and if both are plugged in, whichever is higer (always USB) wins. The charger only charges the battery then, not the rest of the circuit. The 3.3V regulator is also rated for 500mA to power external devices like GPS, LCD screens, etc. Since the power switch is only rated for 120mA, it now controls a fully-rated PMOSFET as the real power switch.
  • Voltage reference - The circuit has a 2.5V series absolute voltage reference attached to ADC pin 8. When connected, pin 8 must be an input on the Logomatic and an output on the board edge connector. If you don't want this, don't short the solder bridge next to the voltage reference. Then you are free to use pin 8 for anything.
  • 11Dof port - This connects to a miniature board with 12 pins and a 3-axis accelerometer, 3-axis gyro, 3-axis magnetometer, and pressure/temperature sensor. The fast sensors, the acc and gyro, are connected to SPI1, while the slow sensors, baro and compass, are connected to I2C1. All the interrupt lines are broken out and connected to ADC pins which are repurposed as timer capture pins. More on this board in another post.
  • Round Corners - PCBs are hard, and the corners can be made arbitrarily sharp. These just don't feel good when you handle them. I got the idea to round off the corners from one of the photos on Laen's blog. These are 0.1" radius corners which happen to be centered on the mounting hole in each corner. I thought there might be a problem with such a thin bit of PCB there, but FR4 is one tough material.
Basically because the Open Hardware license is contagious, when I publish the design, it will be with the same terms that Sparkfun uses. The original Logomatic was designed by C. Taylor and N. Seidle at, and released with a Creative Commons attribution/share-alike 3.0 license.