Sometimes the simplest projects are the best. This is my “Backdoor to Scope Out” project. I use my “Backdoor” connection to interface a computer to FPGAs when a regular bus ( PCI ) isn’t available. What I call the “Backdoor” is just a FTDI 1×6 asynch serial connection at 921,600 baud – pretty much standard in Arduino land – I just use it as a really slow bus bridge into FPGAs. As soon as a regular connection ( PCI for example ) is working, the backdoor connection goes unused. This project repurposes the otherwise dormant Backdoor header for debug to drive state signals out to an oscilloscope using low cost cables. The cable solution I chose was 1/8″ TRRS ( headphone jack ) to RCA Audio-Left,Audio-Right, Composite-Video that was standardized by the Camcorder industry. The cables are cheap ( $10 ) and fairly high quality that go 3′ to 6′ from a compact 1/8″ headphone plug to 3 RCAs that are easily converted to BNC.
The first board converts a 0.100″ Header to a thru-hole TRRS jack. I was shocked and disgusted to discover that “shield” isn’t used for signal return ( Ground ) on the TRRS connector – but instead used for Audio.
The second board converts the 1×6 FTDI header to the “new” 1×6 reversible scope-out header. I wanted to make the connection reversible so that the ~bulky~ 1/8″ plug and socket could go in 1of4 orientations when plugged in to guarantee clearance from other components that may be on a PCB.
The final test for the project was to drive a 50 MHz and 40 MHz square wave onto 2 of the 3 RCA cables over 6 feet and observe final signal integrity and any potential cross talk. End result was great – perfect for FPGA state debugging. Total cost for complete 3 channel solution was under $20. Now I have a robust 3 channel 0.100″ connection to an oscilloscope using readily available cables.