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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
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So we designed those resistors into the V12.6 main board after reading the same posts you read… and that is what everyone has today (see the main board schematic out on my github). ?After a lot pf experimentation, i found the ISO7241 driver specifically designed to buffer/drive/isolate the SPI bus. ? You might have a look at it to see if it works for your projects. ?
Dr.?William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ ? Owner - Operator Big Signal Ranch – K9ZC Staunton, Illinois ? Owner – Operator Villa Grand Piton - J68HZ Soufriere, St. Lucia W.I. Rent it: www.VillaGrandPiton.com
email:??bill@... ?
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Show quoted text
On Mar 25, 2025, at 10:01?AM, Mark J Culross <mjculross@...> wrote:
? 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
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In the T41, there is also crosstalk on the display cable.? In the image below, the shorter ribbon cable does not work.? You need a cable of this type with about half this length to work.? The longer cable has the SPI signals reordered and separated by the ground and power wires.? It does work.? Perhaps it can be longer.? I didn't test that.
?
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Just curious; do you think a piece of UTP CAT 5 or 6 would work?
John, W5JSS On Tue, Mar 25, 2025 at 9:58?AM Terrance Robertson, KN6ZDE via <tmrob4= [email protected]> wrote:
In the T41, there is also crosstalk on the display cable.? In the image below, the shorter ribbon cable does not work.? You need a cable of this type with about half this length to work.? The longer cable has the SPI signals reordered and separated by the ground and power wires.? It does work.? Perhaps it can be longer.? I didn't test that.
?
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Hi Mark-
?
The T41 project has been successful with this device:
?
There was a run of a couple hundred kits which used this device between the Teensy 4.1 and the display.
The kits were successful, and if the operation of the display was a problem, we would have heard about it here.
?
I have updated the T41 Main board to what I am calling the T41-2.? Here is the schematic, which shows the above
buffer inserted in the SPI bus between the Teensy and the BuyDisplay:
?
?
I did my best to orient the Teensy on the pcb such that the route is as short was possible.? There is a connector and
a short cable.? The clock is upped to the maximum which I believe is 45 MHz.? The operation has been flawless.
Note that the T41 kits have a longer path and yet there are no reported issues.
?
Note that there may be an alternative.? The Teensy/Arduino libraries do not expose the GPIO strength and bandwidth settings.
These are claimed to exist on the PJRC Teensy 4.1 web page.
?
So it could be a matter of configuring the SPI GPIOs to higher output power and bandwidth.? I did find a discussion of this at
the PJRC forums.? I'm going to look at this problem when I work on the next version of the Main board.? I may insert
jumpers so the board can be built with or without the buffer chip.
?
--
73 Greg KF5N
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Very interesting...
Jack, W8TEE
On Tuesday, March 25, 2025 at 01:25:53 PM EDT, Greg KF5N via groups.io <greg.electricity@...> wrote:
Hi Mark-
?
The T41 project has been successful with this device:
?
There was a run of a couple hundred kits which used this device between the Teensy 4.1 and the display.
The kits were successful, and if the operation of the display was a problem, we would have heard about it here.
?
I have updated the T41 Main board to what I am calling the T41-2.? Here is the schematic, which shows the above
buffer inserted in the SPI bus between the Teensy and the BuyDisplay:
?
?
I did my best to orient the Teensy on the pcb such that the route is as short was possible.? There is a connector and
a short cable.? The clock is upped to the maximum which I believe is 45 MHz.? The operation has been flawless.
Note that the T41 kits have a longer path and yet there are no reported issues.
?
Note that there may be an alternative.? The Teensy/Arduino libraries do not expose the GPIO strength and bandwidth settings.
These are claimed to exist on the PJRC Teensy 4.1 web page.
?
So it could be a matter of configuring the SPI GPIOs to higher output power and bandwidth.? I did find a discussion of this at
the PJRC forums.? I'm going to look at this problem when I work on the next version of the Main board.? I may insert
jumpers so the board can be built with or without the buffer chip.
?
--
73 Greg KF5N
-- Jack, W8TEE
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This information is incorrect. ?The 74HC125 was the source of horrendous noise generation in the HF band spectra when connected to the SPI bus. ?This is documented in the forum exchanges from fall two years s ago. ?
Dr.?William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ ? Owner - Operator Big Signal Ranch – K9ZC Staunton, Illinois ? Owner – Operator Villa Grand Piton - J68HZ Soufriere, St. Lucia W.I. Rent it: www.VillaGrandPiton.com
email:??bill@... ?
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Show quoted text
On Mar 25, 2025, at 1:25?PM, Greg KF5N via groups.io <greg.electricity@...> wrote:
? Hi Mark-
?
The T41 project has been successful with this device:
?
There was a run of a couple hundred kits which used this device between the Teensy 4.1 and the display.
The kits were successful, and if the operation of the display was a problem, we would have heard about it here.
?
I have updated the T41 Main board to what I am calling the T41-2.? Here is the schematic, which shows the above
buffer inserted in the SPI bus between the Teensy and the BuyDisplay:
?
?
I did my best to orient the Teensy on the pcb such that the route is as short was possible.? There is a connector and
a short cable.? The clock is upped to the maximum which I believe is 45 MHz.? The operation has been flawless.
Note that the T41 kits have a longer path and yet there are no reported issues.
?
Note that there may be an alternative.? The Teensy/Arduino libraries do not expose the GPIO strength and bandwidth settings.
These are claimed to exist on the PJRC Teensy 4.1 web page.
?
So it could be a matter of configuring the SPI GPIOs to higher output power and bandwidth.? I did find a discussion of this at
the PJRC forums.? I'm going to look at this problem when I work on the next version of the Main board.? I may insert
jumpers so the board can be built with or without the buffer chip.
?
--
73 Greg KF5N
|
Any digital bus, by its nature, is going to produce noise.? That's the physics of digital waveforms (high slopes and discontinuities).? This issue is mitigated by applying good EMI design practices including decoupling, board design, and shielding.
This is somewhat of an art and might require multiple prototype passes to achieve.? I was able to make a few simple changes to the T41 boards and the display noise is close to undetectable.? The buffer part was not removed from the design.
What little noise remains certainly can't compete with HF band noise.? The radio also has switching regulators and a Class D audio amplifier.? The radio is working fine and the receiver is a pleasure to operate.
?
Bill, are you saying the T41 kits are unusable?? The displays work great!? I think the biggest problem with the kits is 10 meter operation.? That can be fixed by swapping a couple of parts.
Mark's application may be tolerant of digital noise and the buffer part may be relevant only towards fixing the display robustness.? In other words he may not need to work a lot on EMI issues.
?
--
73 Greg KF5N
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Greg et al:
The TMPS previously had noise induced in the audio chain by the display interface when everything was managed by a single Teensy (touching the screen & changing onscreen objects caused a high-pitched whine in the audio...the audio was also interrupted/delayed by the same actions).? That's the primary reason why I split the functionality across two Teensy processors, as well as spreading/reducing the RAM consumption & spreading/reducing the processor load.? Now that the audio & display are on separate Teensy processors, there is absolutely no detectable noise in the generated audio stream & everything executes in real time, so problem(s) solved.
Mark J Culross KD5RXT
On Tue, Mar 25, 2025 at 1:14 PM, Greg KF5N via groups.io <greg.electricity@...> wrote: Mark's application may be tolerant of digital noise and the buffer part may be relevant only towards fixing the display robustness.? In other words he may not need to work a lot on EMI issues.
?
--
73 Greg KF5N
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“ Bill, are you saying the T41 kits are unusable?? The displays work great! ”
I said no such thing. ?The display worked but the noise from the SPI buffer chip brought the noise level in the receiver up to an intolerable level
I habe no knowledge of Marks application. ?
Dr.?William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ ? Owner - Operator Big Signal Ranch – K9ZC Staunton, Illinois ? Owner – Operator Villa Grand Piton - J68HZ Soufriere, St. Lucia W.I. Rent it: www.VillaGrandPiton.com
email:??bill@... ?
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Show quoted text
On Mar 25, 2025, at 2:14?PM, Greg KF5N via groups.io <greg.electricity@...> wrote:
? Any digital bus, by its nature, is going to produce noise.? That's the physics of digital waveforms (high slopes and discontinuities).? This issue is mitigated by applying good EMI design practices including decoupling, board design, and shielding.
This is somewhat of an art and might require multiple prototype passes to achieve.? I was able to make a few simple changes to the T41 boards and the display noise is close to undetectable.? The buffer part was not removed from the design.
What little noise remains certainly can't compete with HF band noise.? The radio also has switching regulators and a Class D audio amplifier.? The radio is working fine and the receiver is a pleasure to operate.
?
Bill, are you saying the T41 kits are unusable?? The displays work great!? I think the biggest problem with the kits is 10 meter operation.? That can be fixed by swapping a couple of parts.
Mark's application may be tolerant of digital noise and the buffer part may be relevant only towards fixing the display robustness.? In other words he may not need to work a lot on EMI issues.
?
--
73 Greg KF5N
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On Tue, Mar 25, 2025 at 01:19 PM, K9HZ wrote:
but the noise from the SPI buffer chip brought the noise level in the receiver up to an intolerable level
I've been happy with my 4SQRP T41, but I don't have much experience with this.? What's considered an "intolerable" noise level for that radio?
?
Currently, my partially built v12 has a receiver noise floor 48 dB above my 4SQRP unit.? I have enough experience to know something is wrong with my v12 build.
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On Tue, Mar 25, 2025 at 10:20 AM, John Schindler wrote:
Just curious; do you think a piece of UTP CAT 5 or 6 would work?
I think it would.? I was going to test this but didn't want to make up another cable.? I was sweating it with the one I made thinking I'd get the wiring wrong or connect it upside down.
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Can you show how you re-ordered the cable?
If you have the "rainbow" type Insulation Displacement cable and raw connectors it's not too difficult to split the wires, reorder them and then hold them in place with a small bit of tape, and then crimp.? I've done this on floppy disk cables in the past.
I'd like to know the specific re-order that you used which worked.
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On Wed, Mar 26, 2025 at 10:37 AM, ken WA2MZE wrote:
it's not too difficult to split the wires, reorder them and then hold them in place with a small bit of tape, and then crimp
Good to know.? I used the following order:
?
1. SCLK (gray)
2. GND (white)
3. VCC (black)
4. CS (brown)
5. VCC (red)
6. MISO (orange)
7. GND (yellow)
8. MOSI (green)
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I believe the ferrite sleeves made for the ribbon cable solved all my issues.
Tim W4YN
?
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A clam-on ferrite did work for the shorter ribbon cable in the image I posted earlier.? It wouldn't work for a somewhat longer cable.
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I replaced the large clamp on with 2 slim ones and put one on each end. Hardly notice they are there.?
Tim?
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Have you got a block diagram of this/ signal flows.? Its something to contemplate vs. a FPGA and a Teensy.? I’m formulating my next steps and want to get the next step absolutely perfect. ? ? Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ VP2EHZ ? Owner - Operator Big Signal Ranch – K9ZC Staunton, Illinois ? Owner – Operator Villa Grand Piton – J68HZ Soufriere, St. Lucia W.I. Rent it: ? Moderator: North American QRO Group at Groups.IO. Moderator: Amateur Radio Builders Group at Groups.IO. ? email:? bill@... ? ?
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From: [email protected] <[email protected]> On Behalf Of Mark J Culross
Sent: Tuesday, March 25, 2025 3:01 PM
To: [email protected]
Subject: Re: [SoftwareControlledHamRadio] RA8875 display problems with T4.x - a visual study? Greg et al: The TMPS previously had noise induced in the audio chain by the display interface when everything was managed by a single Teensy (touching the screen & changing onscreen objects caused a high-pitched whine in the audio...the audio was also interrupted/delayed by the same actions).? That's the primary reason why I split the functionality across two Teensy processors, as well as spreading/reducing the RAM consumption & spreading/reducing the processor load.? Now that the audio & display are on separate Teensy processors, there is absolutely no detectable noise in the generated audio stream & everything executes in real time, so problem(s) solved. ? ? On Tue, Mar 25, 2025 at 1:14 PM, Greg KF5N via groups.io Mark's application may be tolerant of digital noise and the buffer part may be relevant only towards fixing the display robustness.? In other words he may not need to work a lot on EMI issues.
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The noise floor for the current V12 radio is better than -100 db as measured without the driver chip… My V11 with the buffer chip is nominally -78 db.? That is ok for a test platform (toy level) but intolerable for a modern radio… The new display driver board does not change that -100 db figure because it was designed specifically to buffer SPI signals (in that frequency range with softer on-off switching). ? ? Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ VP2EHZ ? Owner - Operator Big Signal Ranch – K9ZC Staunton, Illinois ? Owner – Operator Villa Grand Piton – J68HZ Soufriere, St. Lucia W.I. Rent it: ? Moderator: North American QRO Group at Groups.IO. Moderator: Amateur Radio Builders Group at Groups.IO. ? email:? bill@... ? ?
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From: [email protected] <[email protected]> On Behalf Of Terrance Robertson, KN6ZDE via groups.io
Sent: Tuesday, March 25, 2025 4:08 PM
To: [email protected]
Subject: Re: [SoftwareControlledHamRadio] RA8875 display problems with T4.x - a visual study? On Tue, Mar 25, 2025 at 01:19 PM, K9HZ wrote: but the noise from the SPI buffer chip brought the noise level in the receiver up to an intolerable level
I've been happy with my 4SQRP T41, but I don't have much experience with this.? What's considered an "intolerable" noise level for that radio? Currently, my partially built v12 has a receiver noise floor 48 dB above my 4SQRP unit.? I have enough experience to know something is wrong with my v12 build.
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-78dB?
My V11 : with HC125 buffer, no shield on display, and 12V switching power supply
?
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K9HZ
jjpurdum