fix(lf): read() acquired with the field OFF (SNIFF) — no tag was ever powered

lf.read() sent CMD_LF_SNIFF_RAW_ADC (passive sniff, reader field OFF), so it captured
only a dead field — a real hardware dump came back flat at 0x7e-0x80 (±1 count of ADC
noise) with no tag modulation, which is why LF identify found nothing. Reader-mode reads
must use CMD_LF_ACQ_RAW_ADC (SampleLF -> field ON) to power the tag and capture its load
modulation.

- read() -> CMD_LF_ACQ_RAW_ADC (field on); sniff() -> CMD_LF_SNIFF_RAW_ADC (field off),
  no longer delegating to read(). Shared _acquire() packs the real lf_sample_payload_t
  (PACKED bitfield samples:30/realtime:1/verbose:1 + cotag byte = 5 bytes) instead of a
  bare uint32.

Tests: read() uses ACQ with a 5-byte payload and correct samples field; sniff() uses SNIFF.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

pm3py/core/lf.py

@@ -111,23 +111,32 @@ class LFCommands:
 
         return result
 
-    async def read(self, samples: int = 12288, verbose: bool = False,
-                   on_progress: ProgressCallback = None) -> dict:
-        """Read LF ADC samples into device buffer."""
-        val = (samples & 0x3FFFFFFF)
+    async def _acquire(self, cmd: int, samples: int, verbose: bool = False) -> dict:
+        """Capture LF ADC samples into BigBuf. Shared by read (reader field ON) and sniff
+        (field OFF). Payload is lf_sample_payload_t — a PACKED bitfield: samples:30, realtime:1,
+        verbose:1, cotag:1 (5 bytes)."""
+        word = (samples & 0x3FFFFFFF)
         if verbose:
-            val |= (1 << 31)
-        payload = struct.pack("<I", val)
-        resp = await self._t.send_ng(Cmd.LF_SNIFF_RAW_ADC, payload, timeout=10.0)
+            word |= (1 << 31)                       # bit 31 = verbose; bit 30 = realtime (0)
+        payload = struct.pack("<IB", word, 0)       # trailing byte carries cotag (0)
+        resp = await self._t.send_ng(cmd, payload, timeout=10.0)
         return {
             "status": resp.status,
             "samples": struct.unpack_from("<I", resp.data, 0)[0] if len(resp.data) >= 4 else 0,
         }
 
+    async def read(self, samples: int = 12288, verbose: bool = False,
+                   on_progress: ProgressCallback = None) -> dict:
+        """Reader-mode LF acquisition: turn the reader field ON and sample the tag's load
+        modulation (this is ``lf read`` — CMD_LF_ACQ_RAW_ADC). Use this before download_samples
+        to read an EM4100/T5577/HID. (SNIFF, field-off, captures nothing from a tag.)"""
+        return await self._acquire(Cmd.LF_ACQ_RAW_ADC, samples, verbose)
+
     async def sniff(self, samples: int = 12288,
                     on_progress: ProgressCallback = None) -> dict:
-        """Sniff LF signal."""
-        return await self.read(samples=samples, on_progress=on_progress)
+        """Sniff LF: reader field OFF, capture an external reader<->tag exchange
+        (CMD_LF_SNIFF_RAW_ADC)."""
+        return await self._acquire(Cmd.LF_SNIFF_RAW_ADC, samples)
 
     async def sim(self, gap: int = 0, data: bytes = b"",
                   on_progress: ProgressCallback = None) -> dict:

tests/test_lf.py

@@ -76,3 +76,27 @@ def test_lf_t55_config_client_side():
     assert len(result["modes"]) == 4
     # No firmware round-trip: SET_CONFIG is write-only and never replies
     t.send_ng.assert_not_called()
+
+
+def test_lf_read_uses_reader_field_acq():
+    # read() must turn the reader field ON (LF_ACQ_RAW_ADC) — SNIFF is field-off and reads nothing
+    t = AsyncMock()
+    lf = LFCommands(t)
+    t.send_ng.return_value = make_response(Cmd.LF_ACQ_RAW_ADC, 0, struct.pack("<I", 98304))
+    result = asyncio.get_event_loop().run_until_complete(lf.read(samples=12288))
+    assert result["samples"] == 98304
+    args, kwargs = t.send_ng.call_args
+    assert args[0] == Cmd.LF_ACQ_RAW_ADC
+    payload = args[1] if len(args) > 1 else kwargs["payload"]
+    assert len(payload) == 5                       # lf_sample_payload_t: 30+1+1 bits + cotag byte
+    word = struct.unpack_from("<I", payload, 0)[0]
+    assert (word & 0x3FFFFFFF) == 12288            # samples field
+    assert (word >> 30) & 1 == 0                   # realtime = 0
+
+
+def test_lf_sniff_uses_field_off():
+    t = AsyncMock()
+    lf = LFCommands(t)
+    t.send_ng.return_value = make_response(Cmd.LF_SNIFF_RAW_ADC, 0, struct.pack("<I", 100))
+    asyncio.get_event_loop().run_until_complete(lf.sniff(samples=4096))
+    assert t.send_ng.call_args[0][0] == Cmd.LF_SNIFF_RAW_ADC