Circuit LED running display
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.
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.
The real time clock is implemented with U10 - PCF8583. This is a clock / calendar / alarm circuit with I2C interface and on-chip 32768Hz oscillator. The PCF8583 contains all necessary counter registers to provide real time clock and date information. Its power consumption is very low (typical supply current is 10 A).
It operates in wide range of supply voltage from 1 to 6V. These features will make it possible to have a real time clock available for a long time using a small lithium battery or even a back-up capacitor. The designed PCB provides both options.