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diff --git a/docs/pinmux.mdwn b/docs/pinmux.mdwn
index 001cc9f83f23e5850de18f513e14a5d38c60496d..5a58ed8e33dd6e88b633ab559cf4e7658fd2f423 100644 (file)
--- a/docs/pinmux.mdwn
+++ b/docs/pinmux.mdwn
@@ -120,6 +120,15 @@ and triaging of faults.
 
 ## Clock synchronisation
 
+Take for example USB ULPI:
+
+<img src="https://www.crifan.com/files/pic/serial_story/other_site/p_blog_bb.JPG"
+width=400 />
+
+Here there is an external incoming clock, generated by the PHY, to which
+both Received *and Transmitted* data and control is synchronised.  Notice
+very specifically that it is *not the main processor* generating that clock
+Signal, but the external peripheral (known as a PHY in Hardware terminology)
 
 
 # GPIO Muxing