Root/cntr/README

Source at commit 36e73c4abb520aa65722ef5e1483bcb8b75bf9f4 created 13 years 6 months ago.
By Werner Almesberger, Moved tools/ out of atrf, in preparation of merge with atusd.
1Arbitrary-precision counter
2===========================
3
4Theory of operation
5-------------------
6
7The arbitrary-precision counter counts clock cycles of a frequency
8source that is assumed to be free from drift. It compares the count
9with the host's system clock. If the system clock is synchronized with
10an accurate NTP reference, measurements with arbitrarily high accuracy
11can be obtained.
12
13In practice, this is limited by the the frequency source's drift and
14the time one is willing to wait. If NTP maintains the system time
15with an accuracy of +/- 100 ms, obtaining measurements with an
16accuracy of +/- 1 ppm would take about 28 hours.
17
18Additional error sources, such as the round-trip time when requesting
19a sample from the microcontroller, are also considered in the accuracy
20calculation.
21
22The counter consists of a board based on a C8051F320 microcontroller
23and the control software on the host. The microcontroller counts
24events in a free-running 16 bit counter that is regularly read and
25extended to 32 bits. The 32 bit counter is periodically queried by
26the host.
27
28The microcontroller's counter can count at a frequency of up to 3 MHz.
29(SYSCLK/4)
30
31In order to protect against transmission errors not detected by USB's
32CRC, which are occur relatively often, each packet is protected by a
33CRC-32 and an inverted copy of the payload. Corrupted packets are
34rejected by the host.
35
36The 32 bit counter wraps around at most once very 21.8 ms. The 32 bit
37counter wraps around at most every 1431 s. The host extends the 32 bit
38counter to 64 bits, and calculates frequency and accuracy from the
39count and the run time of the measurement application.
40
41
42Performing a measurement
43------------------------
44
45To perform a measurement, connect the CNTR board's probe input to the
46clock source and then run the "cntr" application on the host. An
47accuracy goal (in ppm) can be specified on the command line (see
48below).
49
50The host polls the microcontroller every 100 ms and displays the run
51time (in seconds), the measured frequency, and the accuracy achieved
52so far.
53
54Measurements can be stopped by pressing ^C or by specifying an
55accuracy goal. At the end, the total number of events counted and
56communication statistics are displayed.
57
58
59Updating the firmware
60---------------------
61
62The protocol revision and the build date of the firmware of the CNTR
63board can be queried with "cntr -i".
64
65To update the firmware, run
66cntr -r && sleep 1 && dfu-util -d 0x20b7:0xcb72 -D cntr.bin
67

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