From 5dbde0ca20156e12ff59e4f0fce3ef1ce952620c Mon Sep 17 00:00:00 2001
From: Joerg Wunsch <j@uriah.heep.sax.de>
Date: Fri, 13 Sep 2013 21:32:00 +0000
Subject: [PATCH] * stk500v2.c (stk500v2_set_sck_period): Revamp this to match
 the description/pseudo-code in appnote AVR068.

git-svn-id: svn://svn.savannah.nongnu.org/avrdude/trunk/avrdude@1229 81a1dc3b-b13d-400b-aceb-764788c761c2
---
 ChangeLog  |  5 ++++
 NEWS       |  1 +
 stk500v2.c | 72 ++++++++++++++++++++++++++++++++++++------------------
 3 files changed, 54 insertions(+), 24 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index ac7a93a1..0e61c8c2 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2013-09-13  Joerg Wunsch <j.gnu@uriah.heep.sax.de>
+
+	* stk500v2.c (stk500v2_set_sck_period): Revamp this to match the
+	description/pseudo-code in appnote AVR068.
+
 2013-09-13  Joerg Wunsch <j.gnu@uriah.heep.sax.de>
 
 	Submitted by Stephen Roe:
diff --git a/NEWS b/NEWS
index c2808a1f..1983dc58 100644
--- a/NEWS
+++ b/NEWS
@@ -112,6 +112,7 @@ Current:
       - patch #7657: Add ATmega406 support for avrdude using DRAGON + JTAG
       - bug #35474: Feature request: print fuse values in safemode output.
       - patch #7710: usb_libusb: Check VID/PID before opening device
+      - [no-id]: Fix SCK period adjustment for STK500v2
 
   * Keep track of input file contents
 
diff --git a/stk500v2.c b/stk500v2.c
index 9ee51132..bdae5ef6 100644
--- a/stk500v2.c
+++ b/stk500v2.c
@@ -281,6 +281,7 @@ static int stk500v2_paged_write(PROGRAMMER * pgm, AVRPART * p, AVRMEM * m,
 
 static unsigned int stk500v2_mode_for_pagesize(unsigned int pagesize);
 
+static double stk500v2_sck_to_us(PROGRAMMER * pgm, unsigned char dur);
 static int stk500v2_set_sck_period_mk2(PROGRAMMER * pgm, double v);
 
 static int stk600_set_sck_period(PROGRAMMER * pgm, double v);
@@ -2860,37 +2861,60 @@ static unsigned int stk500v2_mode_for_pagesize(unsigned int pagesize)
   exit(1);
 }
 
-/* This code assumes that each count of the SCK duration parameter
-   represents 8/f, where f is the clock frequency of the STK500V2 master
-   processors (not the target).  This number comes from Atmel
-   application note AVR061.  It appears that the STK500V2 bit bangs SCK.
-   For small duration values, the actual SCK width is larger than
-   expected.  As the duration value increases, the SCK width error
-   diminishes. */
+/*
+ * See pseudo-code in AVR068
+ *
+ * This algorithm only fits for the STK500 itself.  For the (old)
+ * AVRISP, the resulting ISP clock is only half.  While this would be
+ * easy to fix in the algorithm, we'd need to add another
+ * configuration flag for this to the config file.  Given the old
+ * AVRISP devices are virtually no longer around (and the AVRISPmkII
+ * uses a different algorithm below), it's probably not worth the
+ * hassle.
+ */
 static int stk500v2_set_sck_period(PROGRAMMER * pgm, double v)
 {
+  unsigned int d;
   unsigned char dur;
-  double min, max;
+  double f = 1 / v;
 
-  min = 8.0 / STK500V2_XTAL;
-  max = 255 * min;
-  dur = v / min + 0.5;
-
-  if (v < min) {
-      dur = 1;
-      fprintf(stderr,
-	      "%s: stk500v2_set_sck_period(): p = %.1f us too small, using %.1f us\n",
-	      progname, v / 1e-6, dur * min / 1e-6);
-  } else if (v > max) {
-      dur = 255;
-      fprintf(stderr,
-	      "%s: stk500v2_set_sck_period(): p = %.1f us too large, using %.1f us\n",
-	      progname, v / 1e-6, dur * min / 1e-6);
-  }
+  if (f >= 1.8432E6)
+    d = 0;
+  else if (f > 460.8E3)
+    d = 1;
+  else if (f > 115.2E3)
+    d = 2;
+  else if (f > 57.6E3)
+    d = 3;
+  else
+    d = (unsigned int)ceil(1 / (24 * f / (double)STK500V2_XTAL) - 10.0 / 12.0);
+  if (d >= 255)
+    d = 254;
+  dur = d;
 
   return stk500v2_setparm(pgm, PARAM_SCK_DURATION, dur);
 }
 
+static double stk500v2_sck_to_us(PROGRAMMER * pgm, unsigned char dur)
+{
+  double x;
+
+  if (dur == 0)
+    return 0.5425;
+  if (dur == 1)
+    return 2.17;
+  if (dur == 2)
+    return 8.68;
+  if (dur == 3)
+    return 17.36;
+
+  x = (double)dur + 10.0 / 12.0;
+  x = 1.0 / x;
+  x /= 24.0;
+  x *= (double)STK500V2_XTAL;
+  return 1E6 / x;
+}
+
 
 static int stk600_set_vtarget(PROGRAMMER * pgm, double v)
 {
@@ -3255,7 +3279,7 @@ static void stk500v2_print_parms1(PROGRAMMER * pgm, const char * p)
     stk500v2_getparm(pgm, PARAM_OSC_PSCALE, &osc_pscale);
     stk500v2_getparm(pgm, PARAM_OSC_CMATCH, &osc_cmatch);
     fprintf(stderr, "%sSCK period      : %.1f us\n", p,
-	  sck_duration * 8.0e6 / STK500V2_XTAL + 0.05);
+	    stk500v2_sck_to_us(pgm, sck_duration));
     fprintf(stderr, "%sVaref           : %.1f V\n", p, vadjust / 10.0);
     fprintf(stderr, "%sOscillator      : ", p);
     if (osc_pscale == 0)