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== Current Operation == === Overview === [http://www.flickr.com/photos/31848713@N00/3430728760/in/photostream/ Overview of the inside of the screen (picture)] [http://www.flickr.com/photos/31848713@N00/3429916211/in/photostream/ Detail of our breadboard (picture)] ==== LED Hardware ==== The sign is currently driven by two boarduinos on a solderless breadboard. One controls the left half of the sign, and the other controls the right half. The old hardware buffer board has 16 power sources that it cycles through in sequence. It goes through each cycle of 16 power sources at about 500 Hz. (So it changes power sources every 1/8000th of a second). Each boarduino shifts bits out along a serial line controlling 192 shift registers. These shift registers act as controllable current-limited drains. The LEDs on the display have their anodes connected to the power sources. Each anode is shared by 192 LEDs. The LEDs on the display have their cathodes connected to the shift registers. Each shift register pin is shared by 16 LEDs. This gives us 3072 LEDs per half of the display, which are arranged into an array 64 LEDs wide by 48 LEDs tall. The boarduinos have connections to the preexisting circuitry to tell which power source is in use at any time, and send the respective data to the shift registers for display. We also have an xbee series 1 (802.15.4) wireless chip onboard so we don't have to open up the sign to talk to it. ==== Software ==== Though we are using arduino compatible hardware, we are using AVR-GCC to compile native code directly for the atmel mega168 chip on the boarduino. The arduino framework does not perform well enough nor allow us good access to the atmega168 integrated chip features such as SPI and TWI. We are using lady ada's arduino bootloader. This acts like an atmel stk500 programmer, and allows us to upload software over the wireless connection. The XBEE wireless chip has virtual transparenet pins configured so we can control the reset pins of the two boarduinos independently, and upload new code to each half of the sign at a time. The two halves talk to each other using the TWI interface on the atmega168. === Pin connections === [[Image:ledscreen-schematic.png|500px]] Here are the pins that need to be connected other than power and ground: * MOSI (PB3) - this sends serial data to the daughterboard * SCK (PB5) - this sends lcock signal to the daughterboard * PB1 - this should be connected to the latch/strobe signal from the old buffer board * PB0 - this should be connected to the latch/strobe signal going to the daughterboard * PD7 - this should be hooked up to the left side of one of the power sources on the buffer board through a voltage divider (and small capacitor to stabilize the signal) to bring the 15 volts down to 5 volts. Do not use the output of the power source, since it's not nearly as stable. It uses the existing STROBE (latch) signal from the buffer board for timing, and reads the state of one of the buffer board's output powers to synchronize where in the sequence of 16 power sources. It captures the latch signal and re-emits it to the daughterboard. It ignores the serial clock and serial data from the buffer board. Unfortunately this has very little processor time to spare since it's spending all its time clocking out the serial data. We're probably limited to very basic patterns on here.
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