Updated Jan 28, 2023 - This is version 2.1.  There is a now V2.2 version of this board. See https://oshwlab.com/k7mdl/v2-1_7-inch_teensy_pcb_copy

This is a motherboard that mounts on the back of a 7" RA8876 touchscreen display from buydisplay.com.  Holes are provided so that this PCB can also plug onto a 4.3" RA8875.  It is an expanded size of the orignal 4.3" board by N0CTL.  This one is a 2 layer board version 2.1.  A version 2.2 was created a few weeks later as a 4 layer to compare noise and fix teh slight hole spacing issue.  There was little to no difference in performance however the V2.2 does have some changed connectors and a couple new small features.

***** I have assembled the first one 1/6/2023 and still testing. The basics are working, 7" mounting holes are off by 0.5mm. The code has been tweaked a bit for some small config changes.  There is a new revision (V2.2) with some minor tweaks.   I do not recommend using a 7805, too much heat, and also running the display at 3.3V off this PCB's 3.3V regulator creates excessive heat, it is better to let the display use its own 5V to 3.3V regulator.  Could opt to run the 3.3V regulator off the main 5V with a jumper.  I use an external 5V DC-DC converter.  Power draw is down to 100-200ma vs almost 1A with the linear regulators.

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SDR project info at https://github.com/K7MDL2/KEITHSDR.  It has the code, BOM, these PCB files, pictures, and Wiki pages.

Extensive information can also be found on the Forum site at https://groups.io/g/keithsdr.

There are 2 SDR variants found on the forum:

1. Simple - ("Keith's SDR").  It uses a lower cost smaller display.  This PCB will not work with the display used by default.  Great choice for low cost and a simple introduction to an Arduino SDR radio using the Teensy 4 CPU, the SGTL5000 audio codec module.

2. SDR_RA887x.  This is my derivation specifically for the RA8876/RA8875 displays and supports this PCB.  It takes advantage of the RA887x display controller capabilities reducing CPU overhead for needed spectrum & waterfall graphics performance.  This motherboard is for the RA887x variant only.  Uses multiple FFTs up to 4096IQ from OpenAudio_Library which is now 32bit floating point.  USB and analog audio connections, compatible with WSJT-X using the Elecraft K3 CAT radio interface.  Can use a variety of RF hardware, I am currently using the RS-HFIQ 4W headless SDR transceiver.  Connection to the RS-HFIQ is by analog Line In and Line Out cables and the Teensy USB Host for serial control. The RS-HFIQ covers 80-10M bands.  There is support for a range of RF side hardware other than the RS-HFIQ and adding something new should be minimal effort.  This includes attenuators, bandpass filters, various Si5351x PLLs, preamp and antenna switching, PTT, mic audio.  

The attached project files includes a full BOM export with some minor corrections, my DigiKey parts order list with costs.  See the Github site for more details and pcitures. 

The display listed in the exported BOM is the 4.3" RA8875.  This PCB is an extension with updates to the original 4.3" sized board (known as the V2.0 board) on this web site from N0CTL. I have a slightly updated 4.3" version 2.1 to be released soon here.  You can use that display with this board.

The 7" Display I use is from buydisplay.com.  Model ER-TFTM070-6 is 1024x600 dots 7 "color tft lcd module display with RA8876 or LT7683 controller board. 

SPI 7"TFT LCD Dislay 1024x600 w/RA8876 OPTL Capacitive Touch Screen (buydisplay.com)

Order options to choose are:

Interface: Pin-Header-Connection-4-wire-SPI

Power Supply: VDD=5.0V

Touch Panel: 7" Capacitive Touch with Controller

MicroSD Card Interface: Pin Header Connecton

Font chip: none

The Teensy 4 related parts are the Teensy 4.1 CPU module and the Rev D SGTL5000 Audio Module.  These plug onto, or solder, to the motherboard.  My GitHub WIKI has a lot of info as well as the forum linked above.

- Mike K7MDL