Lab 11.15 - Runtime `bridge_rx_only` witness using stock TX and gpreg RX counters¶

Objective¶

Determine whether the runtime RX path inside the course PL overlay can observe any samples when the stock AD9361 host TX path is used as the signal source.

The lab separates two possible failure modes that look identical in earlier logs:

the overlay bitstream is loaded correctly and axi_gpreg answers, but the PL RX chain never increments rx_valid_count regardless of what drives TX;
the PL RX chain increments rx_valid_count when a known-good host TX drives the link, but was starved earlier because the course PL TX also was broken.

Using the stock host TX as a witness eliminates the second ambiguity.

Background terms¶

bridge_rx_only: a bridge_txrx_mux overlay variant where TX is strapped to a constant and only the RX datapath from the AD9361 is wired into the PL BPSK chain, removing PL TX as a variable.
host TX: the stock AD9361 DMA TX path driven directly from the Linux host over network IIO — the same cyclic-BPSK burst used in Lab 11.14.
gpreg RX counters: rx_valid_count and received_bits inside axi_gpreg at 0x79040000; non-zero means the PL framer actually processed symbols.
rx_common re-init: an optional write to the AD9361 RX common control register to re-arm the DMA path before capture; controlled by --rx-common-reinit.

Files¶

Path	Purpose
`blocks/block_11_integrated_sdr_project/python/lab_11_15_runtime_bridge_rx_host_tx_probe.py`	load `bridge_rx_only` overlay, run stock host TX, poll gpreg RX counters
`blocks/block_11_integrated_sdr_project/python/runtime_rx_common.py`	shared helper to force RX common control register re-init

Run¶

python blocks/block_11_integrated_sdr_project/python/lab_11_15_runtime_bridge_rx_host_tx_probe.py \
  --bit-bin tmp/bridge_rx_only.wordswap.bit.bin \
  --run-tag live_witness

The script:

uploads and loads bridge_rx_only.wordswap.bit.bin through Linux fpga_manager;
probes axi_gpreg ID/signature to confirm the overlay is alive;
configures the stock AD9361 RX path at 915 MHz / 3.84 MS/s;
optionally forces rx_common re-init if --rx-common-reinit is given;
starts a cyclic BPSK burst from the host TX path;
polls rx_valid_count and received_bits up to --poll-limit times;
stops TX, restores AD9361 state, optionally reboots;
saves a JSON evidence file with all counter snapshots and timing.

Safe first settings (checked-in defaults)¶

Parameter	Default
Center frequency	915 MHz
Sample rate	3.84 MS/s
RF bandwidth	2 MHz
TX attenuation	-50 dB
RX gain	35 dB
Poll limit	128
Poll delay	20 ms
`start_offset`	62

Interpreting the result¶

`rx_valid_count` after TX	Meaning
remains 0	PL RX chain is not receiving samples — overlay or DMAC issue
increments	PL RX chain is alive; previous failure was PL TX starvation

If rx_valid_count stays zero even with a confirmed host TX burst, the blocker is inside the bridge_rx_only RX datapath or the AD9361→PL interface, not in the BPSK modem TX logic.

Evidence collected on 2026-06-23¶

The bridge_rx_only witness run showed rx_valid_count = 0 after both a plain overlay reload and after an rx_common re-init attempt, while the host stock TX was confirmed to be transmitting the BPSK cyclic burst. This proved that the PL RX path itself does not receive samples from the AD9361 under the runtime overlay — irrespective of what generates the RF signal.

Next step: investigate whether the AD9361 → PL sample bus (LVDS or parallel) is properly re-armed after a hot fpga_manager reload.

Report checklist¶

[ ] Record axi_gpreg ID value (confirm 0x4250534B).
[ ] Record tx_valid_count, rx_valid_count, received_bits after each poll.
[ ] Note whether rx_common re-init changed the counter trajectory.
[ ] Attach dmesg tail.
[ ] Confirm host TX was transmitting (BER = 0 from a stock-shell self-check).

Engineering conclusion template¶

The runtime bridge_rx_only overlay loaded and axi_gpreg ID = 0x____.
After ____ poll cycles with stock host TX active at ____ MHz,
rx_valid_count = ____ and received_bits = ____.
RX common re-init was / was not attempted; it did / did not change the result.
Conclusion: the PL RX path is / is not receiving AD9361 samples under the
runtime overlay because ______.

Lab 11.15 - Runtime bridge_rx_only witness using stock TX and gpreg RX counters¶