As I remarked in a recent post, I've nailed down the cause of my pesky REM detector noise issue. Additionally, I started sprucing up the display subsystem, using a DMA channel to significantly reduce the processor load needed. I've taken another couple of steps forward, in that I've implemented a fix to the REM noise issue, and I've refined the display subsystem so that it can easily produce text and primitive graphics; these refinements are shown immediately below.
The video shows the scrolling display in action; since the sensor is pointed at the couch arm, there's not much going on, except when I rotate it.
Implementation details, vaguely
On the display side, this is really only a slight complication of what I outlined previously; there's a local buffer that I draw on, and periodically I set the DMA channel to transmit the contents of that buffer to the display via the serial transmitter peripheral. There's a little bit of trickery translating the signed ADC inputs into the bytes that get painted into the display buffer, but it's overall quite straightforward. A zipped archive of the project files is here.
On the REM detector side, I'd determined that the reason I was getting 'bias noise' every four samples was that the PWM switching transients were making it into the ADC signal, and because the PWM and REM counters were not integer multiples of each other, they would only 'line up' every four samples. To rectify this scenario, I modified the code to make sure that the sample timer is always set up to be an integer multiple of the PWM timer period. I also set up the clocking of the timers to go through the XMEGA's event system, so that I could keep the timers in sync. A plot of the signal below shows no abnormal patterns in the noise, so I think this has taken care of the problem.
Moving Forward again
So, my next step has to be nailing down the REM detector. Specifically, I'm going to slightly modify the hardware so that I can quarter the PWM duty cycle. This way, I can maximize the settling time between the PWM switching time and the ADC sample time. After this is done, I'm going to assess whether this has had the effect of decreasing the noise; either way, I'm going to collect some real-life sleep data, and get cracking on whether I can assemble a filter/classifier that can detect REM.
Of course, this completely avoids the issue of the busted charger circuit; unfortunately, I still haven't secured access to a hot-air rework station...
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| The bottom 3/4 of the display is the REM differential signal level, scrolling from left to right. The numbers on top are the 32-bit integer holding the device's seconds counter (left) and the IR illuimnator level (right). |
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| Not very interested waveform-wise, but a clearer picture. |
Implementation details, vaguely
On the display side, this is really only a slight complication of what I outlined previously; there's a local buffer that I draw on, and periodically I set the DMA channel to transmit the contents of that buffer to the display via the serial transmitter peripheral. There's a little bit of trickery translating the signed ADC inputs into the bytes that get painted into the display buffer, but it's overall quite straightforward. A zipped archive of the project files is here.
On the REM detector side, I'd determined that the reason I was getting 'bias noise' every four samples was that the PWM switching transients were making it into the ADC signal, and because the PWM and REM counters were not integer multiples of each other, they would only 'line up' every four samples. To rectify this scenario, I modified the code to make sure that the sample timer is always set up to be an integer multiple of the PWM timer period. I also set up the clocking of the timers to go through the XMEGA's event system, so that I could keep the timers in sync. A plot of the signal below shows no abnormal patterns in the noise, so I think this has taken care of the problem.
Moving Forward again
So, my next step has to be nailing down the REM detector. Specifically, I'm going to slightly modify the hardware so that I can quarter the PWM duty cycle. This way, I can maximize the settling time between the PWM switching time and the ADC sample time. After this is done, I'm going to assess whether this has had the effect of decreasing the noise; either way, I'm going to collect some real-life sleep data, and get cracking on whether I can assemble a filter/classifier that can detect REM.
Of course, this completely avoids the issue of the busted charger circuit; unfortunately, I still haven't secured access to a hot-air rework station...
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