From c86df8e13eb52aea817c14221d0223fdb08bf8d1 Mon Sep 17 00:00:00 2001
From: whitequark <whitequark@whitequark.org>
Date: Mon, 19 Mar 2018 06:23:12 +0000
Subject: [PATCH] firmware: try to unstuck the I2C bus if it gets stuck.

Fixes #957.
---
 artiq/firmware/libboard_artiq/i2c.rs | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/artiq/firmware/libboard_artiq/i2c.rs b/artiq/firmware/libboard_artiq/i2c.rs
index 819c21c1b..e46b3d0fd 100644
--- a/artiq/firmware/libboard_artiq/i2c.rs
+++ b/artiq/firmware/libboard_artiq/i2c.rs
@@ -59,7 +59,19 @@ mod imp {
             half_period();
             half_period();
             if !sda_i(busno) {
-                error!("SDA is stuck low on bus #{}", busno)
+                warn!("SDA is stuck low on bus #{}, trying to unstuck", busno);
+
+                // Try toggling SCL a few times
+                for bit in 0..8 {
+                    scl_o(busno, false);
+                    half_period();
+                    scl_o(busno, true);
+                    half_period();
+                }
+            }
+
+            if !sda_i(busno) {
+                error!("SDA is stuck low on bus #{} and doesn't get unstuck", busno);
             }
         }
     }