No need for an external crystal, one can use the GPIOs of the Raspberry Pi to generate a clock signal. The library wiring makes it easy and accessible.

Wiring for the Raspberry Pi was written by Gordon Henderson.

In the terminal:

 $ sudo apt-get install wiringpi

For the Raspberry Pi 4 one needs to update the WiringPi to version 2.52:

 $ cd /tmp
 $ wget https://project-downloads.drogon.net/wiringpi-latest.deb
 $ sudo dpkg -i wiringpi-latest.deb

http://wiringpi.com/wiringpi-updated-to-2-52-for-the-raspberry-pi-4b/

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The clock frequency is derived from the 19.2MHz Raspberry Pi clock. One can use GPIO 4 (pin7), GPIO 5 and GPIO 6.
2.4Mhz would be 2400000

Terminal example:

 $ gpio mode 7 clock
 $ gpio clock 7 2400000

Or in C:

 pinMode (pin, GPIO_CLOCK) ;
 gpioClockSet (pin, frequency) ;

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graphic by indiBit.de
License: CC BY-NC-SA 3.0 DE

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http://wiringpi.com/
https://www.raspberrypi.org/forums/viewtopic.php?t=22274

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