USB WORKS!


After 3 weeks, finally got USB on the 18F14K50 to work. Turns out the load capacitors on the crystal need to be symmetric. Asymmetric traces worked with everything else except USB.

Also 4 U assembly language freaks, some of the USB registers are not in the special function register access bank so you need BANKSEL calls.

HOME MADE RADIO WORKS!

Next came getting the radio to work on flight hardware. It was a hard battle over 6 months but full duplex radio with MRF49XA, USB, & miniaturized Marcy 1 boards finally started working. This is truly a ground station you could put inside a phone. 50Hz & 40kbit each way still appears to be the maximum as with XBee.

Be sure to set the analog modes & tristates on all the SPI pins because SPI doesn't even generate interrupts if they're not set properly. Start out at 9600 baud & work up to 86000 from there. The FIFO on the MRF49XA has to be flushed before switching from transmit to receive mode. Switching from transmit to receive required writing an extra PMCREG | 0x0020 not in the reference software.

Also, it doesn't byte align so you need to communicate in fixed packet sizes & reset the sync code scanner between packets. This shows how unoptimized XBees are for the sake of convenience. You could do better by skipping byte alignment, packetization & just sending a continuous stream. This would require always keeping the FIFO full.

Currently the Marcy 1 radios are running at 86207 bps. The maximum is 344827 bps.

HOME MADE RADIO RANGE TEST

How bout a range check for these home made radios?


Ground station & aircraft boards in the field.


Aircraft board hung up for range testing.


Roving ground station.

The Marcy 1 radios seem superior to XBee PRO 900's. They fade gradually like 900Mhz yet don't go as far as the XBee PRO 2.4Ghz. Signal is useful up to 600ft but can reach 800ft if the antennas are carefully aimed. 800ft is the limit of the golf course. This is full duplex of course. Suspect range increases as the modulated bitrate decreases.

Marcy 3 could easily use a dual frequency, full duplex radio made out of dual MRF49XA's. It would get longer range by resending until bandwidth saturation. A single 18F14K50 can control 2 radio chips.

FYI, the MRF49XA is pin compatible with the TRC101 & SI4421. It's a licensed, dirt cheap, standard design.

DEATH OF 72MHZ

The 72Mhz RC transmitter has a bleak future. A new $20 receiver is required for Marcy 1's ground station before we can even spin up the engine. The antenna is always going to wear out & need replacement. 72Mhz recently dropped out on the golf course.

There isn't much advantage to a wireless RC transmitter anymore. A wireless connection for manual flight is fragile & completely useless for indoor vehicles. You can't really move far from the ground station because you need telemetry. A tethered transmitter wouldn't need batteries & recharging. Any commercialization would require a tethered transmitter. Signal quality from a wireless RC transmitter is 1 too many pieces of telemetry.

Unfortunately, the money was already spent on more 900Mhz radios & it would take nonexistent money to now go back to a wired RC transmitter.


The answer is no. You can't solder with photoresist still on.


While Marcy 1 waits for 900Mhz ground station radios, Marcy 2 continues fabrication.



The last of the parts packed into the 900Mhz RC transmitter, Marcy 2 flight computer, & RC receiver. Hoping the 900Mhz can function without a balun in receive mode because baluns are a real pain to build & we suspect they reduce range.


FINAL THOUGHTS ON 2009

It was a tough year like all odd numbered years. While still our hope to someday bring in a real live Air Force heroine to add a little more realism to the Jack Crossfire blog, it doesn't seem likely. Still grateful to have fought back & gotten on better terms with the AF than we were in September.

In 2010 we have a breakthrough aerodynamics concept Marcy 1 & a tail sitter Marcy 2 budgeted, assuming the boss gives us any time off.

Views: 1701

Comment by Andy Geppert on January 1, 2010 at 2:13pm
You are hard to keep up with. More great work, and thanks for sharing the details.
In previous posts (such as this one) regarding radio range, you often provide a graph displaying the kbps through-put over time. How do you grab the real-time through-put for graphing purposes?
Comment by Jack Crossfire on January 2, 2010 at 2:31am
You can't with commercial RC equipment. This is a custom system not for sale.
Comment by Ernesto Pareja on January 3, 2010 at 9:26pm
Hello Jack... Is the ground plane below the MRF49XA mandatory or it can work without it?
Can you please share the MRF49XA circuit values? thank you

Ernesto Pareja
www.dcctrains.xf-s.com
Comment by Jack Crossfire on January 3, 2010 at 10:00pm
The MRF49XA schematic is as described in the datasheet. Suspect the ground plane is required for optimum reception.
Comment by alex stevenson on January 13, 2010 at 9:22pm
thats awesome i was looking for this great work :)
A display stands designer

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