Monday, August 3, 2009

RFM-USB Interface

The photo shows the RFM-USB interface made using double-sided, non-PTH PCB. As can been seen, the top layer components pads are not soldered. This approach result in higher number of vias but simplify soldering of the double-sided board. 100% homebrewable...

During testing, another problem was identified. The RFM12B has a max VCC of 3.8V so I used a 3.3V regulator for it and simple voltage divider on the signals from PIC to the RFM. Then, the output from RFM12B are wired directly to the PIC. Upon measurement, the voltage of the RFM12B output max to about 3.7V. Unfortunately, the PIC input (SDI) has a schmidt trigger thus requires minimum iput of 0.8xVCC. At 5V, that translated to 4V... thus higher than what the RFM12B can deliver. Hmmm... more thinking/decision required:

  1. Run the RFM12B at USB voltage (normally below 5V) which is over the design max but below the absolute max (of 6V).
  2. Replace the RFM12B with RFM12 (meant for 5V) and run the interface at USB voltage.
  3. Run the PIC at 4.2V (minimum for PIC18F2550) via voltage regulator or simple diode voltage drop.
  4. Add level translator on the RFM12B output to bring the output to VCC.
I'm using option 1 for now (using my previous board). The above new board still works since the VCC from USB on my PC is around 4.6V. Thus 0.8x4.6V->3.68V, something the RFM12B still can managed. For actual production, I'll use option 1 and 2, i.e., run everyting at VCC from USB and replaced the RFM12B with RFM12.

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