PWM Logger / Generator
The Protean PWM logger was designed to facilitate the development of hobbyist autonomous vehicle systems whose performance strongly depends on servo data collection and reproduction. The logger has the following default functions, but is fully reprogrammable.
- PWM signal generation for up to 6 channels.
- PMW signal decoding for up to 6 channels.
- Data access over I2C.
The PWM logger is built around Parallax's Propeller (P8X32A-Q44), an 8 core MCU which operates at ~20 MIPS per core. Dedicating each core's processing power to each servo channel offloads a potentially significant burden from your project's main computer. And allows for accurate decoding and generation of PWM signals.
The PWM logger uses the I2C protocol for configuration and operation. It joins the bus as a slave device with the default 7-bit address 0x69. The logger is capable of, and tested with clock speeds of 100KHz and 400KHz but should be capable of higher speeds.
When configuring or using the PWM logger the first thing that your application will have to do is begin with an I2C write addressing the device and writing one byte which will select that register specifically. If you are planning on writing to the selected register you should continue with the current write transaction. Otherwise, end the transaction and follow up with a read.
- 0x00 - Configuration register: Controls the mode of operation of the PWM logger. By default, the value in that register can have the following values.
- 0 -> Echo off, this means the PWM logger will ignore PWM signals on the input channels. Instead it will generate the pulse width in the channel register.
- 1 -> Echo on, this means the PWM logger will measure PWM signals on the input channels, and store the measurement in each channel register.
- 0x0N - Channel register: These registers store the pulse width as a pseudo time from [0, 255]. The pulse duration in seconds for a given channel register value can be obtained with the following operation (value << 10) / clock speed where the clock speed in the normal case is 8 * 10^7 Hz.
- Once a channel register has been selected with a write, you can continue reading or writing multiple bytes as the selected register will be auto incremented. This allows you to collect data from, or write new timings to, all the servo channels in a single I2C transaction.
- When the device is in echo mode the measured pulse widths are stored in each respective channel register.
- When the device has echo mode disabled, the last pulse width in each channel register will be generated on that corresponding channel. That means writing to that register will cause the device to generate that particular pulse width.
- 0x0A - Firmware revision register: This register contains a single byte indicating the revision of the currently flashed firmware.
- 0x0B - Hardware reset register: Writing any value to this register will trigger a hardware reset of the board.
The PWM logger's firmware is written in PASM, the propeller's native language. More information about PASM can be found in the propeller manual.
Building & Flashing
For all platforms you will need to have Parallax's SimpleIDE installed. It provides the necessary toolchain for assembling the source and flashing it to the propeller chip. You will also need a Prop Plug to perform the actual flashing.
Mac / Linux
Once you have installed the SimpleIDE you will need to add the toolchain to your
PATH environment variable.
On MacOS the toolchain is likely to be found in
/Applications. In my case, I was able to add it to my path like so.
On linux, run the following command.
In both cases, you can make the change permanent by adding it to your
.bashrc file (if bash is your shell).
After that, you should be ready to assemble the program and flash the board by using the following.
This will both assemble the program and flash the device. The program will run immediately. Note: You may need to change the device file /dev/ttyUSB0 used in the Makefile to match the device file assigned to the Prop Plug on your system.