The subject of problems with the SPI interface between the T4.x & RA8875 displays comes up quite often on the Teensy forum.? I would like to point out that the SPI interface is not anything that PJRC has much control over (it's baked into the processor that NXP created).? I'm not defending PJRC, just suggesting that any angst should almost certainly be directed to NXP rather than PJRC.
I personally am using an RA8875 display from buydisplay.com in my TeensyMIDIPolySynth (TMPS) project, & my experience with incorporating this display included some significant challenges.? I initially wire-wrapped a prototype of my TMPS & everything worked like a champ.? When I took the design to PCB, I transitioned through multiple (very expensive, with the PCB being 5.5" x 8.5" !!) versions of the PCB (with additional decoupling caps, with additional ground plane, with no ground plane, with two ground planes sandwiching everything else & a very large number of thru-vias stitching them closely together, with shorter SPI signal traces, with orthagonal SPI signal traces, etc.).
I was very frustrated until I found a few posts on the Teensy forum suggesting placing small value resistors (50-100ohm) inline with some of the SPI signals.? So, I cut traces on one of my non-working PCBs & patched in some resistors.? Doing so actually resolved my specific display problems.? I determined (thru experimentation & by observing the signals on my o-scope) that 100ohm resistors resulted in the cleanest SPI signals for my particular setup, so that's what the final PCB version for my TMPS successfully includes.? I am using the SPI interface connected to the RA8875 display at native speeds, with no software slowdown.? I have not had any RA8875 display problems of any kind since.
To me, this is very counter-intuitive: how could a very sloppy wire-wrapped board work just fine, but a very clean looking PCB not allow the display to work ??? Signal ringing & signal cross-talk were the major contributors to my display failures.? I know that there's some controversy with this "solution" of using inline small value resistors, but, for reference, here's a very recent Teensy forum post that visually shows the difference in the quality of the SPI signals before & after the use of inline small value resistors:
For comparison, I'd be interested in testing one of my non-working TMPS PCBs with the K9HZ buffer chip, if that's still an available option.
Mark J Culross
KD5RXT
P.S. As a side note (since the subject of using serial interconnects between processors has also recently been discussed), my TMPS makes use of two Teensy boards: T4.1 runs the display & manages traditional 5-pin DIN serial MIDI, USBmidi, & USBhostMIDI, while a T4.0 manages all of the audio thru a Teensy Audio Adapter.? The two Teensy boards communicate over a shared 500Kbps serial port (settings flow T4.1-to-T4.0, audio/performance status flows T4.0-to-T4.1, & debug info flows both ways).? The device is a 3-voice, multi-waveform, multi-modulator (selectable FM & PM), 12-poly synth, with filtering & envelope control of all generators, where all of the controls are digitally displayed as buttons & sliders on the RA8875 display, switching between & among multiple menu screens, & everything can be adjusted in real time with no degradation of the resulting sound while it is being generated, & no induced delays in the MIDI response.? I am continually impressed with what a beast the T4.x actually is.? MJC
