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  1. If this is your first experience with RS485 I...

    If this is your first experience with RS485 I recommend digging up an old CIrcuit Cellar article titled "The Art and Science of RS485". Google usually turns up a pdf.

    One thing to watch out for is...
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    It depends a lot on your application. I went the...

    It depends a lot on your application. I went the low level route with no file system because I needed higher write speeds.

    When writing data to an SD card you get best performance when writing in...
  3. A MPU6050 should always report 0x68. Since this...

    A MPU6050 should always report 0x68. Since this is derived from the I2C address, it should also require a change to that to get something different.

    Unless it is not a MPU6050. Some other flavor...
  4. One of the things the startup code does is read...

    One of the things the startup code does is read the WHO_AM_I register to make sure that the magic number returned is as expected. Perhaps your board is returning a different number.
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    Since the SPI clock during initialization is...

    Since the SPI clock during initialization is supposed to be under 400KHz, a slow interface isn't going to be a problem at that stage.
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    I didn't have any trouble. Figure 35 shows a...

    I didn't have any trouble. Figure 35 shows a simplified version of the input circuitry including CADIN which is the sampling capacitor with a typical value of 1.5pF. Figure 36 shows the minimum...
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    You are sampling at 50KHz so have some sort of...

    You are sampling at 50KHz so have some sort of filter with a corner frequency of less than 25KHz. The average setting causes the ADC to perform 4 conversions just as fast as it can. The samples are...
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    This is of dubious utility. Sampling a signal...

    This is of dubious utility.

    Sampling a signal four times in quick succession might help if it were the ADC that was the problem. If you have a proper pre-sample filter in place then the only...
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    Empty as in nothing driving the input at all? Of...

    Empty as in nothing driving the input at all? Of course that will be noisy.
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    A charge amplifier is the usual thing to use with...

    A charge amplifier is the usual thing to use with a piezo sensor.
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    I learned a long time ago that electrons are...

    I learned a long time ago that electrons are notional little things that will go where they want to rather than where I think they should go. Details matter and schematics rarely show enough of it....
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    That current is required to meet all...

    That current is required to meet all specifications for that very high speed part. What is really required for your low speed application? And high impedance load? See Fig. 6 which plots output...
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    A couple of questions: Does the optocoupler...

    A couple of questions:

    Does the optocoupler really require 10mA?

    Do you really need optical isolation? (This implies that the receiver doesn't work well with others.)
  14. If you have 100 Ohm termination resistors on each...

    If you have 100 Ohm termination resistors on each end of the cable, then driving 5V into that will require 100mA. Compared to that, everything else is small change.

    The driver will not actually...
  15. Current demand on a RS485 net is usually driven...

    Current demand on a RS485 net is usually driven by the termination resistors. The transceivers don't require enough to matter much.
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    I am a bit concerned with the lack of a delay...

    I am a bit concerned with the lack of a delay from requesting data to reading it. The data sheet is a bit opaque on the conversion time. To sidestep that I would reverse the order of operations in...
  17. No. The charge on a capacitor is C*V. When the...

    No. The charge on a capacitor is C*V. When the ADC samples the signal it connects the analog input to the sampling capacitor.Typical a few pF. (See data sheet for the particular value for your...
  18. Looking at the startup code, I see that the code...

    Looking at the startup code, I see that the code hooked into the early_hook by default disables the device watchdog. So any replacement is going to have to do that in addition to whatever else you...
  19. 1) Disabling interrupts in addByteToTx() just...

    1) Disabling interrupts in addByteToTx() just wastes time since txB_wptr isn't modified anywhere else. Certainly not in another ISR.

    2) The more natural (and faster!) way to write that is:

    ...
  20. Since analogReadContinuous is just a fancy way to...

    Since analogReadContinuous is just a fancy way to read the ADC result register, it can't be the cause of your trouble. The ADC will keep on performing conversions in continuous mode until you stop it...
  21. Pretty much every I/O module has clock control...

    Pretty much every I/O module has clock control that must be enabled before using that module. Those bits are in the SIM_SCGCx registers. The DAC also has to be initialized with things like your...
  22. Why in the world would you serialize the data? I...

    Why in the world would you serialize the data? I could see doing it if you were short on I/O but you aren't. Even at 75MSPS that fast processor is going to be hard pressed to read and do something...
  23. Including the conversion time is a wasted effort....

    Including the conversion time is a wasted effort. The ADC requires a clock input with very low jitter which means that you know (or should know) the timing precisely. Use that clock to trigger the...
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    That 8 Ohm load on your output can't be making...

    That 8 Ohm load on your output can't be making that opamp happy.

    If all you want to do is reduce the amplitude to drive a low level input then all you need are some resistors. But make sure that...
  25. Since you only need to sink current a simple NPN...

    Since you only need to sink current a simple NPN transistor driver is sufficient and robust enough so that the Zener isn't required.
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    You didn't fix it. The only way that can have any...

    You didn't fix it. The only way that can have any hope of working is if there is a connection between the opamp ground and the ground of whatever is driving J1. Which defeats the purpose of the...
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    I am wondering how that circuit is supposed to...

    I am wondering how that circuit is supposed to work.

    The the opamp has no explicit reference for the signal it gets from J1 (MIDI-in). It might be getting it via its output which manages to have...
  28. When I look for a data sheet on the AD8225 IA,...

    When I look for a data sheet on the AD8225 IA, what I find looks nothing like your schematic. The AD8225 is a single device in an 8 pin package with fixed gain and no Rg.

    What are you really using?
  29. 220nF seems a bit small for 30Hz.

    220nF seems a bit small for 30Hz.
  30. Do you have a plot of ADC counts versus input...

    Do you have a plot of ADC counts versus input voltage? Patterns in that might suggest where the problem lies.
  31. The more typical way of doing this would be to...

    The more typical way of doing this would be to use a timer in capture mode. That way it does the pulse counting for you.
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    I used a Teensy to build a rocket data logger...

    I used a Teensy to build a rocket data logger.
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    It depends on the environment. If you are putting...

    It depends on the environment. If you are putting SD cards into the socket only at a nice safe ESD protected workbench, then no. If you are going to let any idiot do it anywhere, then yes, you need...
  34. Another possibility. If you aren't using the...

    Another possibility. If you aren't using the Arduino environment, what are you doing for startup code? The C startup can spend a fair amount of time copying data for initialized global variables and...
  35. There are some exceptions that might get...

    There are some exceptions that might get triggered and have default handlers. The default is fault_isr() which tries to print some info. You can use something else if it isn't providing the...
  36. Teensy uses the GCC compiler so all of the tools...

    Teensy uses the GCC compiler so all of the tools used for that are available including those that tell you about memory usage. (size, readelf, etc) At root a linker script (specific to each device,...
  37. No. It has no input and only output. (A sensor...

    No. It has no input and only output. (A sensor that you can't read is a poor sort of sensor.) Since the data sheet shows CS to output enable and disable times, there should be no conflict with other...
  38. Since the MAX31855 has no data input I fail to...

    Since the MAX31855 has no data input I fail to see what a tristate buffer on MOSI will do for it.

    You can run into problems with devices that use the same pins to support either I2C or SPI. These...
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    The data sheet addresses this. You add a resistor...

    The data sheet addresses this. You add a resistor to limit the fault current.
  40. As stated before, for best ADC performance you...

    As stated before, for best ADC performance you need an external ADC which you can isolate from all of the digital noise present on the MCU chip. That does make some effort to isolate the ADC system...
  41. They are. The Teensy has some extra filtering...

    They are. The Teensy has some extra filtering between the digital and analog power pins.
  42. Which ground? There is an analog ground pin...

    Which ground? There is an analog ground pin provided.
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    How cheap? Compare them to the Analog Devices...

    How cheap? Compare them to the Analog Devices eval board paying particular attention to the power supplies.
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    It depends on the power supplies. If both have...

    It depends on the power supplies. If both have their power output returns connected to safety ground then some of that LED current is going to want to take the route of your ground connection between...
  45. First dig up a copy of "The Art and Science of...

    First dig up a copy of "The Art and Science of RS-485" which appeared in Circuit Cellar Magazine some time ago. A pdf usually turns up in a Google search. 200 feet is fairly short for RS-485 although...
  46. The SDFAT code that I have looked at doesn't...

    The SDFAT code that I have looked at doesn't disable interrupts. At least at the SD interface level. I can't think of any good reason to disable them in any case.

    For a parallel interface I would...
  47. It depends on the library. On the 3.6 I can write...

    It depends on the library. On the 3.6 I can write about the same amount of data (and more) using my own interrupt driven code. (Warning: I use Forth rather than C.) I use the SDHC DMA system while...
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    Some information is missing: 1) Is there a common...

    Some information is missing: 1) Is there a common ground connection that you didn't mention? 2) What about pullup resistors?
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    The Arduino style pin names A0- map to Teensy...

    The Arduino style pin names A0- map to Teensy ports in strange ways. From the schematic they appear spread across all of the GPIO ports. Certainly not all on GPIOC.
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    I am not the person to answer any questions on...

    I am not the person to answer any questions on the Teensy libraries. I skipped those and programmed the hardware directly in Forth. So I have a fair understanding of how the SD interface works (I...
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