Using just a microcontroller with a 5x5 display would require 10 available output pins on the microcontroller. Also, the output pins would have to be capable of sourcing and sinking the required current to drive it. As a worst case scenario with all LED's on and drawing 5mA, the microcontroller would have to source 25mA in total (since only one row or column would be on at one time).

 

When using larger displays, a microcontroller with even more output pins and current drive is needed. We can get around these requirements by using some additional 74 series logic chips to go between the microcontroller and LED matrix.

 

A better way to select each row (or column) would be to use a decoder. For any size up to 8x8, a 1-8 decoder such as the 74LS138/74HC138 or 74LS155/74HC155 would do.

 

 

These logic chips can takes 3 inputs (A, B, C) from the microcontroller to select one of 8 outputs (Y0,..,Y7). For example, sending '000' would select output Y0, and sending '100' would select output Y1, etc. Thus only 4 output ports are needed to control and clear it. Note that this chip uses active-low outputs so when the output is active it corresponds to a low, with all the other outputs kept high. Depending on how you are using this chip, you may need to use an additional chip to invert the output. Also note that the current drive is very low, and so something like a darlington array may be needed.

 

Another option is to use a serial in, parallel out shift register. These work by clocking in 8 bits to set which outputs are on or off. The 74LS164/74HC164 is a basic shift register which will cause the outputs to shift once each time a new bit is clocked in. This can be offset by turning off the selected row or column while the new row or columns data is clocked in.

 

A better option is the 74LS595/74HC595 shift registers, which features an additional storage register so that the new output data can be clocked in without effecting the previous output.

 

 

To use this chip, first you must clock in the required bits using the SER line, with the SCK line used for the clock input. Then once all 8 bits have been clocked in, the RCK line is used to transfer the data from the storage register to the output register. Then with the !G input pulled low to enable the outputs, the clocked in data can be used to control a connected LED matrix row or column. The HC (High Speed CMOS) version has quite a high current drive capability, and can be used to drive a LED matrix directly.