Raspberry Pi DSI Connector Spec

The DSI specification designed by the MIPI Alliance defines a high-speed serial communication interface between a display panel and a multimedia processor such as that on the Raspberry Pi. The purpose of this specification is to standardise the physical layer of the interface to maintain compatibility across manufacturers of active matrix display panels. An advantage of this specification is that it uses the least number of electrical connections, which results in less crosstalk, higher speeds, and lower power consumption.

The success of smart phones such as the Samsung Galaxy Mega indicates a trend towards larger screen sizes and resolutions. However, this also means large amounts of data transferring at high speeds between the display panel and the GPU resulting in higher power consumption. The physical layer, also known as D-PHY, is one of the most advanced data communication designs currently available for mobile devices, which aims to reduce the power consumption usually associated with high-speed data transfer. The interface operates in simplex and duplex modes with any number of data lanes. The system automatically switches between modes as and when required to keep power consumption to minimum. At minimum, the interface uses one clock lane and one data lane, and at maximum, it uses one clock lane and four data lanes. Each data lane can alternate its mode between high speed differential signalling, and low speed single ended signalling.

The Raspberry Pi DSI connector has two data lanes for the display panel. Data lane 0 is bi-directional which allows the processor to read the registers on the display controller. Data lane 1 sends data in one direction from the processor to the display panel.

The diagram above shows that each data lane consists of two electrical conductors which are used together to send the data in a differential mode. Consequently, the positive and negative labelled lines carry different phases of the signal.

The clock pulse generated by the multimedia processor also transmits to the display panel in a differential mode in one direction only. With the pin count being so low, and if you know what to look for, it is possible to figure out the pinout of the Raspberry Pi DSI connector.

Pinout

In the banner photograph above, you can see that there are six thin tracks going to the socket. The first pairs of tracks connect to pin 2 and pin 3 for data lane 1. The second pairs connect to pin 5 and pin 6 for the clock pulse. Finally, the third pair connect to pin 8 and pin 9 for data lane 0. If you have some sharp probes, you could connect them to the solder pads and hook it up to a multi-channel oscilloscope. I managed to hack this part very easily and interface a tablet display; however, my driver needs more work. Packet transmission protocols are always a pain!

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