From d8b3707231c21e8e1121d6cf542444c94191c3a8 Mon Sep 17 00:00:00 2001
From: lkcl <lkcl@web>
Date: Sun, 14 Nov 2021 18:31:10 +0000
Subject: [PATCH]

---
 docs/pinmux.mdwn | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/docs/pinmux.mdwn b/docs/pinmux.mdwn
index 001cc9f83..5a58ed8e3 100644
--- 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
-- 
2.30.2