The easiest way to transmit data with clock using an MCU
The SPI peripheral should be the perfect solution for this. Since SPI output on the serial output is 1 for 1 with the data loaded into the transmit register you should be good to go. The main thing to consider is that you get 8 clocks (if dealing with byte data) @ the 20MHz to have the software get the next byte from memory ready to be output. 

You may want to investigate. Some MCU's have internal DMA channels built in which can be used to hose data from memory to a peripheral. This would be an excellent way for your application.