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datasheet A6281Power Connections
The V+ and GND pins power both the LED and the control chip. ShiftBrites require up to 60mA per module when all LEDs are active. The supply voltage should be kept between 5.5 and 9 volts. I have had good results with 6V and 7.5V power supplies.
As is true of most digital electronics, if you connect power backwards or short contacts with metal objects or other modules on the chain, you WILL destroy the logic chip. Please triple-check all cables and make sure the modules are not in contact with anything conductive BEFORE applying power.
Data In / Data Out
The DI (Data In) pin carries the actual control information into the ShiftBrite. It is the input to an internal 32-bit shift register. Every time data is shifted into the controller, the binary value on the DI pin is placed in Bit 0 of the shift register, and the value in Bit 31 overflows out the DO (Data Out) pin to the next ShiftBrite in the chain. Data is shifted in using MSB (most significant bit first).
Clock In / Clock Out
The CI (Clock In) pin controls the shifting process. Each time the CI pin is sent to logic high and low, data is shifted into the DI pin and out of the DO pin. The CI signal is passed through the ShiftBrite to the CO (Clock Output) pin, so the next ShiftBrite can receive the bits from the DO line.
Latch In / Latch Out
The LI (Latch Input) pin causes the ShiftBrite to accept whatever is in its shift register as a new command. If you send the LI pin high and then low after 32 clocks, the first ShiftBrite in the chain has all new data from the DI pin. The second ShiftBrite contains whatever was already in the first ShiftBrite, and so on. To command all ShiftBrites in a chain, you must toggle the LI pin after you have shifted data to all ShiftBrites; 32 clock cycles times the number of ShiftBrites in the chain. The LI pin passes through to the LO (Latch Output) pin.
Enable In / Enable Out
The EI (Enable Input) turns the entire chain on and off. If it is sent to logic high, then it will blank all ShiftBrites. When EI is low, all ShiftBrites will display the colors specified previously. The EI pin passes through to the EO pin.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
PWM Counter 0 PWM Counter 1 PWM Counter 2 0 X
Dot Correction 0 Clock
Mode X Dot Correction 1 X X X Dot Correction 2 X ATB ATB 1 X
Each color has 10 bits of the 32 bit command word, as shown in the above diagram. The 10 bits allow PWM (pulse width modulation) control of LED brightness to 1024 possible levels. This is only true if the remaining 2 bits are set to zero. Otherwise, the ShiftBrite enters a command mode that allows current control and other parameters. This should always be set as shown in the code below, unless you have a specific reason to do otherwise. Additional information is available at the bottom of page 7 in the A6281 datasheet.
Essentially, when Bit 30 is 1 the ShiftBrite enters command mode, the last row of the table above. You can set a 7-bit current control value for each LED, 0 to 127 corresponds to about 33% to 100% power. The Clock Mode setting lets you change the PWM frequency or use an external source; it is normally set to 00 which is an 800KHz internal PWM (please refer to the A6281 datasheet for other settings if needed). The bits marked "X" have no effect on the device. The bits marked "ATV" are for Allegro internal testing, but have the side effect of stopping the output buffers. That means if you accidentally write an ATB due to noise, you must write to each ShiftBrite's command register to be able to rewrite the next one.
Since the control mode can set the ShiftBrite to unusable modes, it is a good idea to write the control register often. Most of my code writes it each time the color values are written.
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