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Old Nov 16, 2012, 07:02 PM
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HMC5843 notes

It's no longer made & like most modern parts, only had a 3 month manufacturing run. It was ordered long ago. You collect enough to build a single run of your product, write the driver as fast as possible, & don't plan on using it anymore.

It's really hard to get working. 1st, the internal pullups aren't good enough to run it higher than 10Hz. It needs external pullups for any useful rate.

2nd, it requires SCK to be low during all SDA transitions. Most I2C devices let you get away with changing SDA while raising SCK.

3rd, the required voltage stability is insane. It needs under 50mA voltage ripple on DVDD & AVDD. The tolerance is slightly higher on AVDD. If AVDD is too unstable, I2C doesn't work at all. If DVDD is too unstable, it doesn't take any readings. To coexist with the 900Mhz radio & aircraft noise, it's going to take a serious, heavy LRC circuit & possibly a LDO regulator.

Despite all this, the only time it worked was when powered directly from an active 3.3V regulator. Powering it with a resistor or diode didn't work.


The update rate has been really slow. The 50Hz rating is optimistic. 10Hz is more realistic for any of the register settings.


It's probably going to end up replaced by a MAG3110FCR1, but the driver is written & working.


Also, it took 1 day to get the radio to work on the Marcy 3 board, even though it was the exact same layout as Marcy 1. It was an untested software change & the fact that full duplex is extremely complicated.
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Last edited by Jack Crossfire; Nov 17, 2012 at 04:14 AM.
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