From f0397a117fb7f6a5fb6b09f514fadb69fc5c2575 Mon Sep 17 00:00:00 2001
From: "Robin P. Clark" <robin48gx@gmail.com>
Date: Mon, 29 Jul 2013 12:35:04 +0100
Subject: [PATCH] filtering low pass, naive implementation compared to
 up-binary-scaled

---
 embedded_c_book/fo_filter/fo_filter.c | 69 +++++++++++++++++++++++++++
 1 file changed, 69 insertions(+)
 create mode 100644 embedded_c_book/fo_filter/fo_filter.c

diff --git a/embedded_c_book/fo_filter/fo_filter.c b/embedded_c_book/fo_filter/fo_filter.c
new file mode 100644
index 0000000..01b752c
--- /dev/null
+++ b/embedded_c_book/fo_filter/fo_filter.c
@@ -0,0 +1,69 @@
+#include <stdio.h>
+
+/*
+ * compile with: gcc fo_filter.c -o fo_filter 
+ * run as      : $ ./fo_filter
+ *
+ * shows how the standard lp first order filter, give a steady signal
+ * truncates it and gives a low result
+ *
+ */
+short int threeup_filter, pa2_an2;
+short int naive_filter;
+double fp;
+
+#define INT16   short int
+INT16 filter16(INT16 var, int depth);
+#define ADRES 0x1FC
+
+main()
+{
+
+    int i;
+    INT16 var,fvar;
+
+    var = ADRES;
+
+    for (i = 0; i < 150; i++) {
+
+        fp = (fp * 7.0 + (double) ADRES) / 8.0;
+
+        naive_filter = (naive_filter * 7 + ADRES) / 8;    // old filter 
+
+        threeup_filter = (((threeup_filter << 3) - threeup_filter) >> 3) + ADRES;
+
+	fvar = filter16(var,3);
+
+        printf(" iteration: %d  naive_filter = %x  threeup_filter = %x  fvar = %x fp = %lf\n", i, naive_filter, threeup_filter, fvar, fp);
+    }
+}
+
+
+
+
+
+#define BINARY_FILTER_SCALE 8
+/* with  BINARY_FILTER_SCALE set to 8, depth valid between 2 and 7 */
+/* with a depth of 2 what we are saying is
+ *  * result = (old_result * 3.0 + new_val ) / 4.0)
+ *   * with a depth of 3
+ *    * result = (old_result * 7.0 + new_val ) / 8.0)
+ *     *  */
+INT16 filter16(INT16 var, int depth)
+{
+
+    long v = var, v2;
+    static long oldv;
+
+    printf(" v = %lx oldv = %lx: ", v, oldv);
+    v <<= BINARY_FILTER_SCALE;  // scale up
+    printf(" v = %lx : ", v);
+    v2 = (oldv << depth) - oldv + v;
+    v2 >>= depth;
+    oldv = v2;
+    printf(" v = %lx oldv = %lx ", v, oldv);
+
+    return (v2 >> BINARY_FILTER_SCALE);
+
+
+}