Added notes for PCB Rev.A

PCB_History.md

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+## Rev A
+
+Prototype.
+
+### Issues
+
+1.  **U2 - Incorrect pinout**
+
+    * Traces to pins 5-8 should be cut
+    * Connect U2-5 to MOSI (U1-17)  
+    * Connect U2-6 to SCK (U1-19)  
+    * Connect U2-7 to VBat  
+    * Connect U2-8 to 5V
+
+*   **U22/U20 - Layout error**
+
+    U20 and U22 share SER outputs with U1-4, this configuration is not usable.
+
+    * Trace between U22-9 and U20-9 should be cut.
+    * U21-10 connection to ground should be cut.
+    * Connect U22-9 to U21-10.
+
+*   **U2 - CS floating**
+
+    CS is not pulled high and is floating when not driven by U1. May interfere with flashing via ICSP.
+
+    Connect U1-1 to 5V via 10K resistor.
+
+*   **ICSP Header - Incorrect footprint used**
+
+    Pinout not compatible with standard ICSP header.
+
+*   **R4 & R5 - incorrect value**
+
+    Use 270-330 Ohm instead of stated 10K.