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67 lines
2.4 KiB
Plaintext
67 lines
2.4 KiB
Plaintext
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Arbitrary-precision counter
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===========================
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Theory of operation
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-------------------
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The arbitrary-precision counter counts clock cycles of a frequency
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source that is assumed to be free from drift. It compares the count
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with the host's system clock. If the system clock is synchronized with
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an accurate NTP reference, measurements with arbitrarily high accuracy
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can be obtained.
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In practice, this is limited by the the frequency source's drift and
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the time one is willing to wait. If NTP maintains the system time
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with an accuracy of +/- 100 ms, obtaining measurements with an
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accuracy of +/- 1 ppm would take about 28 hours.
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Additional error sources, such as the round-trip time when requesting
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a sample from the microcontroller, are also considered in the accuracy
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calculation.
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The counter consists of a board based on a C8051F320 microcontroller
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and the control software on the host. The microcontroller counts
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events in a free-running 16 bit counter that is regularly read and
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extended to 32 bits. The 32 bit counter is periodically queried by
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the host.
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The microcontroller's counter can count at a frequency of up to 3 MHz.
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(SYSCLK/4)
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In order to protect against transmission errors not detected by USB's
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CRC, which are occur relatively often, each packet is protected by a
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CRC-32 and an inverted copy of the payload. Corrupted packets are
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rejected by the host.
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The 32 bit counter wraps around at most once very 21.8 ms. The 32 bit
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counter wraps around at most every 1431 s. The host extends the 32 bit
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counter to 64 bits, and calculates frequency and accuracy from the
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count and the run time of the measurement application.
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Performing a measurement
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------------------------
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To perform a measurement, connect the CNTR board's probe input to the
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clock source and then run the "cntr" application on the host. An
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accuracy goal (in ppm) can be specified on the command line (see
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below).
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The host polls the microcontroller every 100 ms and displays the run
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time (in seconds), the measured frequency, and the accuracy achieved
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so far.
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Measurements can be stopped by pressing ^C or by specifying an
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accuracy goal. At the end, the total number of events counted and
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communication statistics are displayed.
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Updating the firmware
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---------------------
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The protocol revision and the build date of the firmware of the CNTR
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board can be queried with "cntr -i".
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To update the firmware, run
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cntr -r && sleep 1 && dfu-util -d 0x20b7:0xcb72 -D cntr.bin
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