The 4 digit backpack provides quick and easy control of four 7-segment digits and decimal points, via a serial protocol interface. The colon and degree LED dots are not connected in this design.
The device will operate with a power supply (Vdd) range of 3 to 5.5 volts. The ICs used are rated for an absolute maximum of 6 volts for Vdd.
To communicate a display item to the device, two sequential bytes of data are transferred, using the Latch, Clock, and Data signals in an SPI protocol transfer. Here is an example:
The latch signal must be low during the serial shift, and raised after the serial shift is complete, to transfer the new data to the display driver.
- The first byte specifies which digits should be selected for illumination. It is interpreted as a bit-mask, so multiple digits could be selected at the same time.
- The second byte is also a bit-mask; it specifies which segments should be illuminated, in all the digits that are currently selected. Note this important constraint; this device cannot display different segments in different digits at the same instant in time. To display an arbitrary 4-digit number, a strobe technique must be used: very rapidly, one digit is illuminated at a time. This is covered in more detail in the demonstration.
The following diagrams illustrate the exact byte values necessary for the respective digit and segment control.