EPROM Display using ULN2308A microcontroller

Due to university and work it has been a while since the last post. But I just completed a little project that’s worth posting.
Several old 27C256 EPROMS were lying around unused. So I thought about a purpose for them. As I also had some 8×8 LED matrices, a little animated display came to mind.Oscillator
With each frame consisting of 8×8 pixels the 32kByte EPROM can hold 4096 frames. Each byte holds one line of the display, eight bytes one frame.
The lower 3 addressbits of the EPROM have to by switched synchronously with the corresponding line on the display. This is achieved by wiring them to a 3-to-8-decoder (74*238) which in turn switches the lines. As up to eight LEDs can light up at once per line. To handle the current an ULN2308A darlington driver is used.
EPROM DisplayThe columns are directly controlled by the data-output of the EPROM. To drive the LEDs a 2N2907 transistor is used.
The clock is generated by a crystal oscillator circuit consisting of a 32768 Hz crystal and an inverter gate.
The 32kHz squarewave from the oscillator is then divided by a 12-stage ripple counter (4040). The seventh to ninth stage are used for the line-addressing and are wired to the A0-A2 inputs of the EPROM and to the A,B,C inputs of the 74*238.
EPROM Display PCBThe next three stages are connected to a DIP-switch. The output of the switch leads to the clock input of another 4040. This way the frame-rate is selectable from 32fps, 16fps and 8fps.
Only fourteen addresspins of the EPROM are used, the fifteenth can be set to high or low via a jumper. For more Detail: EPROM Display using ULN2308A microcontroller

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