ChibiCube hardware explained

ChibiCube hardware explained

1 In this article

By this article, we will show schematic of ChibiCube in order to understand how it works and hot it has been designed.

2 Toward ChibiCube schematic

As mentioned in the previous article, ChibiCube addresses all its LEDs through the combination of anode and cathode signals. Each floor of the cube is configured as common anode, while the various cathode signals are separately connected to the outputs of the Shift Register. Setting the anode voltage to a value equal or greater than the high logical threshold of our SR, for the same principle illustrated in the tutorial Meeting SPI, we can address single LEDs of the floor. To make it simple, consider a cube 4x4x4: the diagram of a single floor would appear as in Fig.1.

ChibiCube plane schematic
Fig.1 – A schematic of a plane from a 4×4 LED cube.

This schematic is different from the one shown in SPI tutorial in some points:

  • The SR is a 16 bit one and is a LED driver. Output current is fixed choosing R-EXT and we don’t need for a resistor on each LED;
  • To make schematic simple, LEDs are oriented as a 4×4 matrix;
  • The common anode is addressed by MCU thought a P-MOS, instead being connected directly to power supply;

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Introducing ChibiCube

Introducing ChibiCube

1 About

ChibiCube is a 3D LED matrix 8x8x8 composed by monochromatic LEDs individually addressable: that means not only every single LED could be in a ON or OFF state but everyone could be set at different light intensity.

2 Inception

In December 2012, students Ciro Domenico Pagano from University of Salerno, Antonio Galano from Federico II University of Naples and Vincenzo Brillante from University of Sannio start their training at Naples site of ST Microelectronics.

In order to understand uses of ChibiOS and STM32 MCUs, their Tutor Giovanni Di Sirio suggested the design of a small 4x4x4 LED cube. This first version was made on a prototyping board and driven by a STM32F407 Discovery using parallel wiring.

Months later, the cube was engineered in a new version 8x8x8 upgradable to 16x16x16 connecting 4 PCBs together.

ChibiCube prototype
Fig.1 – A prototype of a 4x4x4 LED cube.

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Meeting SPI

Meeting SPI

1 The Serial Peripheral Interface

1.1 Introducing SPI

Communicating by a serial bus instead of a parallel one could be very useful when we need to send out a big number of words or fixing a constraint on wires. The Serial Peripheral Interface bus, also known as SPI bus, is a synchronous, serial communication peripheral which communicates in full duplex mode using a master-slave architecture with a single master.
That means we have always a single master and one or more slaves. The communications is bi-directional (i.e. data flows from master to slave but also from slave to master). Since this bus is synchronous, the master (that handles and manages the communication) provides also a clock on a specific line (known as SCK or CLK) so the data is send-out/captured on a clock edge.

Both master and slave send-out and capture 1 bit per clock period. Using other words for each clock period master send and receive a bit and the same happens for slave: to do this are required two separated line (MISO and MOSI).

As there may be more than a slave, the master needs for a slave selector and this is implemented using an additional line for each slave. In this way only the slave having its select line lowered actually communicates with master. Because of that this line is know as Slave Select (SS) as well as Chip Select (CS) (or also as nCS, nSS where n means negate). Nothing has been said on data size as it depends on hardware. However, most common word sizes are 8-bit, 12-bit or 16-bit.

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