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Mar 13, 2013, 02:20 AM
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Death of wifi


The 1st flight test showed a 1000Hz IMU with 2000Hz PWM was extremely stable. The IMU was lowpass filtered at 1/4 bandwidth. There's no vibration problem with this indoor quad. Unfortunately, the rate of I2C crashes was too high for any useful flying.

It was 1 year since the embedded Wifi stack was implemented, at great difficulty. There was a lot of nostalgia for the time when it was being implemented.

You always long for a past time, no matter how difficult it was, being 1 year younger, having 1 more year to go. As impractical as it was for wireless video, it sure was neat to see it stream video for the 1st time. No-one else ever did anything nearly as ambitious with the TCM8230 & STM32F4.

The I2C crashes weren't known until after wifi was running. Then came the discovery that hardware I2C didn't work when wifi was on.

1 year later, it was routinely transmitting 2.5 megabits when the optical flow was in debug mode. You still can't buy a radio module with that bandwidth.

It was finally discovered that pulling up USB+ with a resistor was enough to crash I2C. It didn't matter if all the USB interrupts were disabled. There is a silicon bug which makes USB host interfere with I2C above a certain frequency. Maybe it's a conflict in the AHB bus.

Being a 1 off aircraft with only 1 mission, there's no point in banging on wifi anymore. A breakout board for I2C would be too heavy & take too long to design. The roving networks modules would be too heavy.

Will the investment in wifi ever be recovered? A ground station bridge between wifi & 900Mhz is now proposed, as big a rewrite as any other idea. The only reason it wouldn't be a standalone wifi module on the aircraft is weight, but it would save a battery & be sexier if it was airborne. Converting between a ground station & aircraft module is just a matter of soldering 1 wire. It needs just enough reliability to start & stop the motor.
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