docs: add architecture diagram

README.md

@@ -21,6 +21,36 @@ Entropy comes from FPGA ring oscillators whose sampled timing jitter is XOR-comb
 - Per-second trials in Global Consciousness Project format (200-bit XOR-masked sums per generator, plus full-rate sums), with Stouffer Z and network variance
 - Raw ring captures and frequency measurement for characterization
 
+```mermaid
+flowchart LR
+    subgraph FPGA["FPGA: Cyclone 10 LP, 120 MHz from a PLL on the SAMD21's 48 MHz clock"]
+        direction LR
+        subgraph GEN["4 generators"]
+            direction TB
+            RO["90 ring oscillators<br/>3 taps XORed per ring"] --> SMP["2-flip-flop sampling"]
+            SMP --> SX["spatial XOR<br/>of all rings"]
+            SX --> TX["temporal XOR<br/>of 1-8 bits"]
+        end
+        SMP --> HM["health monitors<br/>repetition count, adaptive<br/>proportion, dead rings"]
+        SX --> HM
+        TX --> SEL["healthy generators<br/>g0-g3 and combined XOR"]
+        HM -->|healthy| SEL
+        SEL --> ST["entropy stream<br/>4 KB FIFO"]
+        SEL --> STATS["statistics<br/>per-trial counts"]
+        SEL --> AMP["bias amplifier<br/>2 paths, 3 stages"]
+        SEL --> GCP["GCP trials<br/>per second"]
+        GEN -.-> CAP["raw capture,<br/>frequency counter"]
+        ST & STATS & AMP & GCP & CAP & HM --> REG["SPI registers"]
+        PT["WiFi module<br/>pin pass-through"]
+    end
+    REG <-->|SPI 8 MHz| MCU["SAMD21 firmware"]
+    MCU <--> PT
+    PT <--> NINA["NINA-W102<br/>WiFi and BLE"]
+    MCU --> USB["USB serial"]
+    NINA --> HTTP["HTTP on its own<br/>access point"]
+    NINA --> BLE["BLE GATT service"]
+```
+
 The USB, HTTP and BLE API is documented in [arduino/strix/API.md](arduino/strix/API.md).
 
 ## Command-line client