v6 Copper Plan: U18 V_BCKP (pin 36) decoupling fix

- pin 36 is V_BCKP on /3V3 (always-on), deliberately not /3V3_GNSS:
  RTC + ephemeris survive when U21's switched rail is off
- no /3V3 cap existed within 19 mm of the pin; spare Cu4 (100nF,
  already /3V3-GND) reassigned to sit at pin 36
- C123 clarified: U18 has only two VCC pins; C123 is the U21-output
  third 100nF, not a VCC-pin cap

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YJ4eAB6zjQT49RVwpgfKdt

03-Architecture-Hardware/2026-09-02_Drogheda_v6-Copper-Plan_Placement-EMC-Guide.html

@@ -162,7 +162,9 @@ a{color:var(--plane)}
 <tr><th>Ref</th><th>Value</th><th>Belongs to</th><th>Placement</th></tr>
 <tr><td>C119</td><td>10 µF</td><td>U21 input (SW_5V)</td><td>at the LDO input pin</td></tr>
 <tr><td>C120 C124</td><td>10 µF ×2</td><td>U21 out → U18 VCC</td><td>one at U21 output, one at U18's main VCC entry</td></tr>
-<tr><td>C121 C122 C123</td><td>100 nF ×3</td><td>U18 VCC pins</td><td>one per supply pin, at the pin</td></tr>
+<tr><td>C121 C122</td><td>100 nF ×2</td><td>U18 VCC pins 33/34</td><td>one per supply pin, at the pin</td></tr>
+<tr><td>C123</td><td>100 nF</td><td>U21 VOUT, beside C120</td><td>U18 has only two VCC pins — C123 is the rail-entry third</td></tr>
+<tr><td>Cu4</td><td>100 nF</td><td>U18 pin 36 V_BCKP (/3V3!)</td><td>V_BCKP rides the always-on 3V3, not 3V3_GNSS — that is deliberate (RTC + ephemeris survive when U21 is switched off). It needs its own /3V3 cap at the pin; C120–C124 are all on /3V3_GNSS and cannot serve it. Repurpose the spare Cu4.</td></tr>
 <tr><td>C144 C145</td><td>100 nF · 100 pF</td><td>U18 VCC_RF · L21 bias tee</td><td>at L21's feed side — the antenna supply injection</td></tr>
 <tr><td>C143</td><td>100 nF RF</td><td>J12 RF line (DC block)</td><td>inline in the 50 Ω run between L21 tap and J12</td></tr>
 <tr><td>D27</td><td>ESD9L (≤0.5 pF)</td><td>J12</td><td>at the connector, stub-less tap on the RF line</td></tr>
@@ -286,7 +288,9 @@ a{color:var(--plane)}
 <tr><td>Q6 R2 C101</td><td>MMBT3904 · 10k · 22 µF</td><td>K1 coil side</td><td><span class="d d2">🟠</span></td><td>relay driver + coil supply cap</td></tr>
 
 <tr class="own"><td colspan="5">U18 — ZED-F9P GNSS (+U21 LDO · J12)</td></tr>
-<tr><td>C121 C122 C123</td><td>100 nF ×3</td><td>U18 VCC pins, one each</td><td><span class="d d1">🔴</span></td><td>per-pin decouplers</td></tr>
+<tr><td>C121 C122</td><td>100 nF ×2</td><td>U18 VCC pins 33/34, one each</td><td><span class="d d1">🔴</span></td><td>per-pin decouplers</td></tr>
+<tr><td>C123</td><td>100 nF</td><td>U21 VOUT beside C120</td><td><span class="d d2">🟠</span></td><td>third 100 nF — rail entry, not a third VCC pin</td></tr>
+<tr><td>Cu4</td><td>100 nF</td><td>U18 pin 36 V_BCKP — on /3V3</td><td><span class="d d1">🔴</span></td><td>only /3V3 cap in the corner; backup rail stays alive when U21 is off</td></tr>
 <tr><td>C120 C124</td><td>10 µF ×2</td><td>U21 output · U18 VCC entry</td><td><span class="d d1">🔴</span></td><td>LDO stability + local bulk</td></tr>
 <tr><td>C119</td><td>10 µF</td><td>U21 input pin</td><td><span class="d d1">🔴</span></td><td>LDO input</td></tr>
 <tr><td>C3 C40</td><td>47 µF · 22 µF</td><td>SW_5V feed near U21</td><td><span class="d d2">🟠</span></td><td>upstream bulk</td></tr>
@@ -360,7 +364,7 @@ a{color:var(--plane)}
 
 <tr class="own"><td colspan="5">Panel · sensors · leftovers</td></tr>
 <tr><td>Cu3</td><td>100 nF</td><td>U3 SHT40 VDD pin</td><td><span class="d d1">🔴</span></td><td>sensor supply</td></tr>
-<tr><td>Cu4</td><td>100 nF</td><td>— spare (ex-TMP102)</td><td><span class="d d3">🔵</span></td><td>reassign to any bare VDD pin or delete</td></tr>
+<tr><td>Cu4</td><td>100 nF</td><td>reassigned → U18 pin 36 V_BCKP</td><td><span class="d d1">🔴</span></td><td>see the GNSS group — it is already on /3V3–GND, so a board move is all it takes</td></tr>
 <tr><td>C99</td><td>2.2 nF</td><td>U17/U12 area rail bypass</td><td><span class="d d3">🔵</span></td><td>verify its role in the sch before final spot</td></tr>
 <tr><td>R41</td><td>10k</td><td>U3/U16 I2C area</td><td><span class="d d3">🔵</span></td><td>pull-up strap</td></tr>
 <tr><td>R3 D1</td><td>120R · LED</td><td>P9 pins</td><td><span class="d d3">🔵</span></td><td>R3 at the jack</td></tr>
@@ -390,5 +394,5 @@ a{color:var(--plane)}
 <li><b>Bench items carried forward:</b> measure U21's 3V3_GNSS rise (F9P ramp window), verify U25's EP against the TI DDA drawing, and the DzG1 jumper check on the fabbed v5.5 boards.</li>
 </ol>
 
-<p class="sub" style="margin-top:36px">EinarBoard HBOX v6 · Helgen · updated 04-Sep-2026: master proximity table (§8) from the live netlist; sch↔pcb designators verified in sync (P1–P12, 0 mismatches).</p>
+<p class="sub" style="margin-top:36px">EinarBoard HBOX v6 · Helgen · updated 05-Sep-2026: U18 V_BCKP (pin 36, /3V3) gets Cu4; C123 clarified as U21-output cap; §8 rows corrected.</p>
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