I am trying to get an accurate timing of a given sample (1-PPS
transition). To do so, a marker is injected in the AD9831 datastream in
the B210 FPGA. When this tag is detected in the UHD source block, a tag
is created in the stream:
for (item=0;item<noutput_items;item++) // check all samples
{if (ch==0) // check if channel 0
{if (in[item]==_tag_pps_int) // check marker detection
{add_item_tag(0, // Port number
nitems_written(0) + item, // Offset
pmt::string_to_symbol("1PPS"), // Key
pmt::from_double(3.14159)); // Value
...
Displaying the item number when the marker is detected, I get things
like 444331/524288 or 364347/524288 (item/noutput_items) which seems
realistic.
Downstream the flowchart, after many other processing blocks, I check
for the tag in the window of samples that were just transferred
std::vector<gr::tag_t> PPS_tag;
get_tags_in_window(PPS_tag,0,
0, // abs_start,
noutput_items, // uint64_t abs_end);
pmt::string_to_symbol("1PPS")) ;
if (PPS_tag.size() != 0)
{ printf("\n** PPS_tag received: %ld (%ld/%d):
message=%.9f",PPS_tag.size(), noutput_items,
to_double(PPS_tag[0].value));
...
and there I *always get* PPS_tag.offset=1.
If on the emitter side I create two tags separated by 42 i.e.
add_item_tag(0,
nitems_written(0) + item,
pmt::string_to_symbol("1PPS"),
pmt::from_double(3.14159));
add_item_tag(0,
nitems_written(0) + item + 42,
pmt::string_to_symbol("1PPS"),
pmt::from_double(3.14159));
then the receiver indeed detects PPS_tag.size() = 2 but still
both PPS_tag[0].offset and PPS_tag[1].offset are equal to 1 !
I am obviously misunderstanding the behviour of tag.offset: can anyone
enlighten me on how to identify the index of the sample carrying the
tag ?
Thanks, JM
