![\"badge\"](\"http:\/\/www.daveakerman.com\/wp-content\/uploads\/2013\/11\/badge.jpg\"){kind=icon}
<\/a><\/p>\nThese displays typically have a USB socket for charging and programming, and appear to a PC or Pi as a virtual serial port. They come with Windows drivers and software to download messages to the device, the intention being that you download your message then unplug and run the display from its internal battery. Messages are stored internally in flash memory so the device can be powered off and it will retain the message.<\/p>\n
For my purposes, I want to be able to send a message from a Pi, and have the device display that message (scrolling if necessary) until I replace the message with another one. I can then get the Pi to display the current tracker status – e.g. “All OK”, or “No GPS Lock”, etc.<\/p>\n
So the first job was to get the device running on my Windows PC. The device installed as a serial device (the common Prolific PL2303 driver) and the supplied software successfully sent messages to it:<\/p>\n
So with the software working, the next job was to have a look at what it was sending to the device. For this I downloaded a serial monitor program from HHD Software<\/a>. This is an excellent program and runs for 14 days in trial mode with nag screens. For the above message and settings it showed this packet being sent, at 1200 baud:<\/p>\n Perhaps surprisingly, the “hello world” text doesn’t appear. In fact, this is because the PC is rendering the text and sending it as a 1-bit bitmap pattern. By sending different messages with different parameters (brightness, scroll speed, direction and “loop” which is a marquee effect) I gleaned the following about the protocol:<\/p>\n The parameter section can be adjusted as follows:<\/p>\n The bitmap packets are 11 bytes each. \u00a0The first byte of the first such packet represents the first 8 dots starting at the top-left of the image being sent, with a “1” representing a lit LED. \u00a0Bit 7 in that byte is the top-left dot, and bit 6 is the one to its right, etc. \u00a0The next byte in the packet is for the next 8 dots\u00a0to the right of the first 8, assuming the image is wider than 8 bytes. \u00a0in other words, the image is scanned one row at a time from top to bottom, and is sent in 11-byte packets. \u00a0The smallest possible image then would consist of 1 bitmap packet, representing 11 rows (the display is 11 rows high) of 8 dots each.<\/p>\n So, with the protocol understood, I needed to port this functionality to Linux on the Raspberry Pi. \u00a0I investigated some Linux programs that can render text as suitable binary dot patterns, such as the strangely named “toilet” program, but the resulting images weren’t as easy to read as ones generated from the Windows program. \u00a0So, I opted for a 2-stage approach; I wrote a Delphi Windows program to build a nice clear character set and then copied the result into a C program on the Pi that can use it to render text before sending it to the display. \u00a0To make the job slightly easier, I had the Delphi program generate actual C source code, like so:<\/p>\n <\/p>\n I put this in a .h file and included it in a new program that:<\/p>\n Initially I did this with the badge connected to the Pi’s USB port. \u00a0The badge appears as \/dev\/ttyUSB0 and it’s a simple task to open that port, set the baud rate, switch off output processing (we’re sending binary data so we don’t want CR\/LF conversions etc!) and to send the data. \u00a0However, this particular badge only displays the message once, and then switches to “charge mode” where it dims the display and shows a battery charge graphic; it only shows the message continuously when unplugged from USB! \u00a0This behaviour is not useful for any application that wants to update the message and then display it for a while before changing the message.<\/p>\n So, I took the display apart to see what I could find:<\/p>\n Unsurprisingly, there’s a Prolific USB-serial converter, with the serial Rx and Tx pins connecting to Tx and Rx on the LPC1113\u00a0ARM Cortex-Mo processor. \u00a0There’s also a Microchip LiPo charge controller, and of course a large matrix of SMD LEDs.<\/p>\n The first thing I wanted to do was bypass the USB interface, since it’s ugly to have a large USB plug sticking out the side of a fairly small display. \u00a0Pin 1 on the PL2303 is the Tx pin that sends data to the CPU, so I carefully lifted that pin with a scalpel and soldering iron. \u00a0Conveniently, the track goes to a relatively large pad labelled “Rx” so I soldered a wire to that for connection to the serial Tx line on the Pi GPIO header. \u00a0With this and a GND wire connected I could happily program the badge from the Pi. \u00a0Finally, I connected 5V from the Pi to the 5V USB line on the badge (the pad for a missing D4 component was ideal for this).<\/p>\n However, there was a problem. \u00a0As mentioned earlier, the badge’s firmware “knows” whether the device is externally powered or not. \u00a0It does this by taking the USB 5V line, passing it through a potential divider to drop to about 3V, and then takes that to a digital input pin on the ARM processor. \u00a0The firmware then does one of the following 2 things, both undesirable, depending on the state of that pin:<\/p>\n So the second scenario is better, but means that every time the Pi changes the message, there’s a delay whilst the startup logo is shown. \u00a0However, a combination of the above would be good – pretend that USB is connected so the message appears quickly and then, before it stops scrolling and enters charge mode, pretend that USB is disconnected.<\/p>\n To do this, I removed the potential divider and connected the ARM’s input pin directly to a GPIO pin on the Pi. \u00a0Here’s the modified board with all 4 wires (5V, 0V, Serial Data, GPIO) connected:<\/p>\n<\/a><\/p>\n<\/a><\/p>\n\n
\n
\n
struct<\/span> TCharacter\r\n{\r\n char<\/span> Character;\r\n int<\/span> Width;\r\n int<\/span> Pattern[11];\r\n};\r\n\r\nstruct<\/span> TCharacterSet\r\n{\r\n int<\/span> Count;\r\n struct<\/span> TCharacter Characters[95];\r\n} CharacterSet = {95,\r\n ' '<\/span>, 4, 0x000, 0x000, 0x000, 0x000, 0x000, 0x000, 0x000, 0x000, 0x000, 0x000, 0x000,\r\n '!'<\/span>, 4, 0x000, 0x000, 0x004, 0x004, 0x004, 0x004, 0x004, 0x004, 0x000, 0x004, 0x000,\r\n '\"'<\/span>, 3, 0x000, 0x002, 0x002, 0x002, 0x000, 0x000, 0x000, 0x000, 0x000, 0x000, 0x000,\r\n '#'<\/span>, 9, 0x000, 0x000, 0x050, 0x050, 0x0FC, 0x028, 0x028, 0x07E, 0x014, 0x014, 0x000,\r\n '$'<\/span>, 7, 0x000, 0x008, 0x008, 0x03C, 0x00A, 0x00A, 0x01C, 0x028, 0x028, 0x01E, 0x008,<\/pre>\n\n
<\/a><\/p>\n\n