From 9e7662e9eacc7aac26660b1382813a6384d85bc0 Mon Sep 17 00:00:00 2001
From: Frank Paynter <paynterf@gmail.com>
Date: Sat, 15 May 2021 15:11:01 -0400
Subject: [PATCH 1/2] modified SetSampleTime() to set the internal SampleTime
 variable to zero for an input argument of zero, without modifying ki or kd. 
 This forces Compute() to generate a new output value each time it is called,
 and allows the PID engine to be synchronized with an external timing source

---
 PID_v1.cpp | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/PID_v1.cpp b/PID_v1.cpp
index cb6637c..f6bfb80 100644
--- a/PID_v1.cpp
+++ b/PID_v1.cpp
@@ -130,6 +130,8 @@ void PID::SetTunings(double Kp, double Ki, double Kd){
 
 /* SetSampleTime(...) *********************************************************
  * sets the period, in Milliseconds, at which the calculation is performed
+ * 15 May 2021 gfp - a value of zero will force Compute() to generate a new output every time it is called
+ * but avoids the 'divide-by-zero' problem
  ******************************************************************************/
 void PID::SetSampleTime(int NewSampleTime)
 {
@@ -139,7 +141,11 @@ void PID::SetSampleTime(int NewSampleTime)
                       / (double)SampleTime;
       ki *= ratio;
       kd /= ratio;
-      SampleTime = (unsigned long)NewSampleTime;
+      //SampleTime = (unsigned long)NewSampleTime;
+   }
+   else if (NewSampleTime == 0)
+   {
+     SampleTime = (unsigned long)NewSampleTime;
    }
 }
 

From a30df926fd1e041cc9059e94f0f9a1cba1acf9f5 Mon Sep 17 00:00:00 2001
From: Frank Paynter <paynterf@gmail.com>
Date: Sat, 15 May 2021 15:32:31 -0400
Subject: [PATCH 2/2] Updated README.txt to include information about synching
 PID engine to external timing sources

---
 README.txt | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/README.txt b/README.txt
index 3f2fb63..ab13b04 100644
--- a/README.txt
+++ b/README.txt
@@ -9,3 +9,7 @@
    http://brettbeauregard.com/blog/2011/04/improving-the-beginners-pid-introduction/
 
  - For function documentation see:  http://playground.arduino.cc/Code/PIDLibrary
+
+ - To synchronize PID::Compute with an external timing source such as a TIMER ISR, 
+	call SetSampleTime() with an argument of zero. This will force Compute to generate a
+	new output value each time it is called.