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error bound. - cntr/tools/cntr/cntr.c (measure): use the middle of a round-trip interval for the sample's nominal time - cntr/tools/cntr/cntr.c (measure): error bound is only +/- RTT/2, not +/- RTT |
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Arbitrary-precision counter =========================== Theory of operation ------------------- The arbitrary-precision counter counts clock cycles of a frequency source that is assumed to be free from drift. It compares the count with the host's system clock. If the system clock is synchronized with an accurate NTP reference, measurements with arbitrarily high accuracy can be obtained. In practice, this is limited by the the frequency source's drift and the time one is willing to wait. If NTP maintains the system time with an accuracy of +/- 100 ms, obtaining measurements with an accuracy of +/- 1 ppm would take about 28 hours. Additional error sources, such as the round-trip time when requesting a sample from the microcontroller, are also considered in the accuracy calculation. The counter consists of a board based on a C8051F320 microcontroller and the control software on the host. The microcontroller counts events in a free-running 16 bit counter that is regularly read and extended to 32 bits. The 32 bit counter is periodically queried by the host. The microcontroller's counter can count at a frequency of up to 3 MHz. (SYSCLK/4) In order to protect against transmission errors not detected by USB's CRC, which are occur relatively often, each packet is protected by a CRC-32 and an inverted copy of the payload. Corrupted packets are rejected by the host. The 32 bit counter wraps around at most once very 21.8 ms. The 32 bit counter wraps around at most every 1431 s. The host extends the 32 bit counter to 64 bits, and calculates frequency and accuracy from the count and the run time of the measurement application. Performing a measurement ------------------------ To perform a measurement, connect the CNTR board's probe input to the clock source and then run the "cntr" application on the host. An accuracy goal (in ppm) can be specified on the command line (see below). The host polls the microcontroller every 100 ms and displays the run time (in seconds), the measured frequency, and the accuracy achieved so far. Measurements can be stopped by pressing ^C or by specifying an accuracy goal. At the end, the total number of events counted and communication statistics are displayed. Updating the firmware --------------------- The protocol revision and the build date of the firmware of the CNTR board can be queried with "cntr -i". To update the firmware, run cntr -r && sleep 1 && dfu-util -d 0x20b7:0xcb72 -D cntr.bin