Implements a simple precise timestamping facility for ADALM Pluto SDR platform, using a timer register built into the ARM Cortex-A9 processor core.
The timer register runs on the CPU clock, at 666,666,687 Hz divided by 2, and constantly counts up from zero unless reset by a write the timer register. This program never writes to the timer register. The register is 64 bits wide, so it will roll over after about 877 years.
If you include this in a larger program there are just two APIs you need to worry about.
This function must be called once before any other API call.
If you're already using /dev/mem and mmap to access registers,
and you probably are, you will probably want to share the open
/dev/mem you already have and copy the mmap code from this
API instead of calling it.
Call this function to read the timestamp from hardware.
This program can be built as a standalone demonstration program to run on the Pluto. For example:
/usr/bin/arm-linux-gnueabihf-gcc --sysroot=/home/abraxas3d/pluto-0.38.sysroot -std=gnu99 -g -o pluto_timestamp pluto_timestamp.c -Wall -Wextra -DMAIN -DLOCAL_DMA_ROUTINES
scp pluto_timestamp root@pluto.local:/tmp
ssh root@pluto.local /tmp/pluto_timestamp
-DMAIN enables the main program. If you're integrating this code
with a larger program, leave this out.
-DLOCAL_DMA_ROUTINES enables the program to implement the
read_dma() and write_dma() functions. If you're integrating
this code with a program that already has these functions, leave
this out.
There are a number of ways you can use the timestamps provided by this code. The example main program uses them to measure the duration of a specific section of code, by taking a timestamp before and another timestamp after, and simply subtracting and scaling by COUNTS_PER_SECOND.
Another obvious application would be to control a loop so as to terminate after a specified time. Take a timestamp at the start, and add the desired duration in units of 1/COUNTS_PER_SECOND seconds. Then each time through the loop, take a timestamp and compare it to that computed sum. When the timestamp value exceeds the sum, it's time to end the loop.
The timestamps could also be used for logging purposes, and for any other purpose you like.
The hardware counter updates every 3ns, but that doesn't mean your timestamps are that accurate or that precise.
As a point of reference, two consecutive calls to
get_timestamp() with nothing in between takes 0.29 microseconds
on my Pluto. Any function calls you make between timestamps
will add additional overhead, of course.
Another issue is that the CPU clock is probably not locked to whatever you're doing in hardware, such as a DSP sample clock. The oscillators on a stock Pluto are not very accurate in frequency. They are reasonably stable at room temperature, but you can expect them to drift with ambient temperature. As a result, you can't expect these timestamps to work for synchronous signal processing on the Pluto. But they should be pretty good for general timing and debugging purposes.