FreHD Revisions

[mudmin]

Does anyone have any info on what has changed in the various FreHD revisions. I have 10x 3.01 boards which differ slightly from the BOM that was included in the zip file. Someone mentioned that one revision had a backwards transistor footprint.

I guess what I'm trying to figure out is:
Is there anything "wrong" with the revision I have or any special precautions that need to be taken with this pcb. This version came (as 2.14) from http://www.vecoven.com/trs80/trs80/download.html

 

[hornbetw]

The footprint for Q1 is not "backwards". There are 2 footprints (different manufacturers) for some reason. They have the Drain and Source swapped on the transistor body. When Fred added Q1, he tied the Gate to the Source instead of +3V3. That keeps Q1 from turning on. If you cut the trace for the Gate and jumper that pin to +3V3, Q1 will work as intended. As noted here (in post #11), you can just remove Q1/R15 and jumper Q1 Drain to Source and the FreHD works fine. That's because the output of an SD card is LVTTL (3.3V) which is compatible with the TTL (5V) input on the PIC. The reason putting Q1 in "backwards" works, is due to the "Body Diode" that exists in all FETs. When you turn Q1 around, the Body Diode passes the TTL LO through Q1 and blocks the LVTTL HI from coming through. R15 pulls the SDI Line to +5V, effectively passing the HI through Q1. If you read this thread around post #10, you'll get some more discussion of the problem. My guess is Fred originally built and tested the FreHD with the transistor that has the Source and Drain that are "backwards" to the footprint he used in the PCB layout. That would have made Q1 function as I just described. Sorry this is so long winded.

Tom

 

[mudmin]

No need to apologize. That's fantastic information. I need to digest it and look at those links. Thank you so much.

 

[mudmin]

Your linked thread cleared everything up. THANK YOU.