# Source: https://learn.adafruit.com/delorean-time-circuit/circuit-trickery.md

# DeLorean Time Circuit

## Circuit Trickery

![](https://cdn-learn.adafruit.com/assets/assets/000/001/308/medium800/lcds___displays_Address.jpg?1396770866)

These 4-digit displays can be assigned one of eight fixed I2C addresses via solder jumpers on the back. But the time circuit needs&nbsp;_nine_&nbsp;displays.&nbsp;A few possibilities were considered, including driving the one extra display “manually” with shift registers, or use a software I2C library and split the displays among multiple I2C buses.&nbsp;Either would require lots of library code changes and some intense concentration, but I was hit with a massive sinus headache at the time and really didn’t want to think about it.

Instead, exploiting the fact that we need just one way, write-only access to use the displays, I used a simple hardware hack to split the I2C bus to communicate with one row of three displays at a time (and saving some code by repeating the same addresses in each row).&nbsp;The I2C data line fans out to all the displays as normal, but the clock feeds the enable lines of a 74HC138 3-to-8 line decoder, and the microcontroller can then select which output line forwards the clock signal. The data on the other I2C buses is ignored without the corresponding clock.

![](https://cdn-learn.adafruit.com/assets/assets/000/001/309/medium800/lcds___displays_Schematic.png?1396770879)

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