mirror of
git://projects.qi-hardware.com/ben-wpan.git
synced 2024-12-24 03:01:08 +02:00
67 lines
2.4 KiB
Plaintext
67 lines
2.4 KiB
Plaintext
|
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
|