Dot matrix LED display

Posted on Feb 22, 2013

The device comprises two parts: LED control board and LED display board. The two PCBs are designed to fit together one behind the other using two sets of dual row connectors and 4 spacers. One of this connector is used for the electrical connections, while the other is only used as a mechanical connecting element. The core of the device is microcontroller PIC18F252 (U9). It controls all the functions of the device, generates the overall algorithm to control the LED matrix. LEDs are connected in matrix 40x7.

Dot matrix LED display
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Dot matrix LED display - image 1
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The columns tie together the cathodes of the LEDs and rows tie the LEDs anodes. The LED matrix is controlled dynamically in row by row. To safe space and number of components, the LEDs are driven with specialized LED driver STP16CP05 (U101-U103), produced by ST Microelectronics. Each of these IC contain 16-bit serial-in, parallel-out shift register, latch register and 16 constant current output channels. Outputs are open drain type, allowing connection of a load supplied with up to 20V supply voltage. The constant current for all outputs varies from 5 to 100 mA and is set from an external resistor (R115-R117). In this application, the three LED drivers are connected in a cascade and controlled from the microcontroller over SPI protocol. The microcontroller sends a 48-bit word, controlling one row at the time. The 40 LSB represent LED states (1-On, 0-Off) in the row and control their cathodes. The 7 MSB control the anodes through the 7 driver transistors (VT101 - VT107). The 40th bit remains unused. The microcontroller sends the 48-bit word every 1 ms. There are 7 cycles to display each row plus an extra blank cycle, used to process temperature measurements. Thus, the refresh rate of the display is 125Hz. To control the display brightness is used the "outputs enable" (OE) pin. Each row cycle begins with logical 0 on the OE pin (outputs are enabled). The duration of an enabled signal changes depending on the desired...

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