I initially was bit-banging the SPI protocol using arbitrary pins on the ATMega2560, but was quite dis-satisfied with the performance. The display was very laggy and slowed down the entire experience.
By switching to use the hardware SPI support on the ATMega2560 (using pins 51 and 52 for MOSI and SCK signals respectively) the performance improvement was spectacular. On the UNO, the hardware pins are 11 and 13 for MOSI and SCK respectively.
I am using a modified version of the Adafruit graphics library and the Adafruit ST7735 driver for my experimentation.
I found a couple of problems which I don't know if I can attribute to the Adafruit libraries or to differences with the specific LCD panel I have in hand. I will have to order one of the Adafruit versions to determine which it is. I found that the color mappings were reversed between RED and BLUE and also between YELLOW and CYAN. If I can obtain a version of the Adafruit display I will test again and report my findings.
I set up my Si5351 clock generator and the TFT display as a three frequency signal generator. There is still some UI work to be done to select between the frequencies to be changed by the rotary encoder, but here is the general idea. I am reusing my previously published rotary encoder bits in this little project.
I will have the ability to independently set each of the clocks on the Si5351. At the moment the code sets the initial frequency of each clock and then moves them all when the rotary encoder turns. I plan to publish the code here when I have it all working satisfactorily.
Performance of the display is now very good, even when updating all three frequencies with the rotary encoder simultaneously. This might be a good candidate for a little Arduino Nano or other minimal Arduino board to allow packaging this up into a useful signal source.
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