diff --git a/src/stk500.c b/src/stk500.c
index cb270036..27334297 100644
--- a/src/stk500.c
+++ b/src/stk500.c
@@ -91,8 +91,6 @@ int stk500_getsync(PROGRAMMER * pgm)
   int attempt;
   int max_sync_attempts;
 
-  /*
-   * get in sync */
   buf[0] = Cmnd_STK_GET_SYNC;
   buf[1] = Sync_CRC_EOP;
   
@@ -119,11 +117,12 @@ int stk500_getsync(PROGRAMMER * pgm)
       usleep(50*1000);
       stk500_drain(pgm, 0);
     }
+
     stk500_send(pgm, buf, 2);
-    stk500_recv(pgm, resp, 1);
-    if (resp[0] == Resp_STK_INSYNC){
+    resp[0] = 0;
+    if(stk500_recv(pgm, resp, 1) >= 0 && resp[0] == Resp_STK_INSYNC)
       break;
-    }
+
     avrdude_message(MSG_INFO, "%s: stk500_getsync() attempt %d of %d: not in sync: resp=0x%02x\n",
                     progname, attempt + 1, max_sync_attempts, resp[0]);
   }
@@ -204,15 +203,14 @@ static int stk500_chip_erase(PROGRAMMER * pgm, AVRPART * p)
   }
 
   if (p->op[AVR_OP_CHIP_ERASE] == NULL) {
-    avrdude_message(MSG_INFO, "chip erase instruction not defined for part \"%s\"\n",
-            p->desc);
+    avrdude_message(MSG_INFO, "%s: chip erase instruction not defined for part \"%s\"\n",
+            progname, p->desc);
     return -1;
   }
 
   pgm->pgm_led(pgm, ON);
 
   memset(cmd, 0, sizeof(cmd));
-
   avr_set_bits(p->op[AVR_OP_CHIP_ERASE], cmd);
   pgm->cmd(pgm, cmd, res);
   usleep(p->chip_erase_delay);
@@ -745,8 +743,8 @@ static int stk500_loadaddr(PROGRAMMER * pgm, AVRMEM * mem, unsigned int addr)
   if (lext != NULL) {
     ext_byte = (addr >> 16) & 0xff;
     if (ext_byte != PDATA(pgm)->ext_addr_byte) {
-      /* Either this is the first addr load, or a 64K word boundary is
-       * crossed, so set the ext addr byte */
+      /* Either this is the first addr load, or a different 64K word section */
+      memset(buf, 0, 4);
       avr_set_bits(lext, buf);
       avr_set_addr(lext, buf, addr);
       stk500_cmd(pgm, buf, buf);
@@ -782,13 +780,12 @@ static int stk500_loadaddr(PROGRAMMER * pgm, AVRMEM * mem, unsigned int addr)
 
   if (stk500_recv(pgm, buf, 1) < 0)
     return -1;
-  if (buf[0] == Resp_STK_OK) {
+  if (buf[0] == Resp_STK_OK)
     return 0;
-  }
 
-  avrdude_message(MSG_INFO, "%s: loadaddr(): (b) protocol error, "
+  avrdude_message(MSG_INFO, "%s: stk500_loadaddr(): (b) protocol error, "
                   "expect=0x%02x, resp=0x%02x\n",
-                  progname, Resp_STK_INSYNC, buf[0]);
+                  progname, Resp_STK_OK, buf[0]);
 
   return -1;
 }
@@ -808,19 +805,20 @@ static int stk500_paged_write(PROGRAMMER * pgm, AVRPART * p, AVRMEM * m,
 
   if (strcmp(m->desc, "flash") == 0) {
     memtype = 'F';
-  }
-  else if (strcmp(m->desc, "eeprom") == 0) {
+    a_div = 2;
+  } else if (strcmp(m->desc, "eeprom") == 0) {
     memtype = 'E';
-  }
-  else {
+    /*
+     * The STK original 500 v1 protocol actually expects a_div = 1, but the
+     * v1.x FW of the STK500 kit has been superseded by v2 FW in the mid
+     * 2000s. Since optiboot, arduino as ISP and others assume a_div = 2,
+     * better use that. See https://github.com/avrdudes/avrdude/issues/967
+     */
+    a_div = 2;
+  } else {
     return -2;
   }
 
-  if ((m->op[AVR_OP_LOADPAGE_LO]) || (m->op[AVR_OP_READ_LO]))
-    a_div = 2;
-  else
-    a_div = 1;
-
   n = addr + n_bytes;
 #if 0
   avrdude_message(MSG_INFO, "n_bytes   = %d\n"
@@ -828,7 +826,7 @@ static int stk500_paged_write(PROGRAMMER * pgm, AVRPART * p, AVRMEM * m,
                   "a_div     = %d\n"
                   "page_size = %d\n",
                   n_bytes, n, a_div, page_size);
-#endif     
+#endif
 
   for (; addr < n; addr += block_size) {
     // MIB510 uses fixed blocks size of 256 bytes
@@ -875,13 +873,13 @@ static int stk500_paged_write(PROGRAMMER * pgm, AVRPART * p, AVRMEM * m,
                       progname, Resp_STK_INSYNC, buf[0]);
       return -4;
     }
-    
+
     if (stk500_recv(pgm, buf, 1) < 0)
       return -1;
     if (buf[0] != Resp_STK_OK) {
-      avrdude_message(MSG_INFO, "\n%s: stk500_paged_write(): (a) protocol error, "
+      avrdude_message(MSG_INFO, "\n%s: stk500_paged_write(): (b) protocol error, "
                       "expect=0x%02x, resp=0x%02x\n",
-                      progname, Resp_STK_INSYNC, buf[0]);
+                      progname, Resp_STK_OK, buf[0]);
       return -5;
     }
   }
@@ -902,19 +900,20 @@ static int stk500_paged_load(PROGRAMMER * pgm, AVRPART * p, AVRMEM * m,
 
   if (strcmp(m->desc, "flash") == 0) {
     memtype = 'F';
-  }
-  else if (strcmp(m->desc, "eeprom") == 0) {
+    a_div = 2;
+  } else if (strcmp(m->desc, "eeprom") == 0) {
     memtype = 'E';
-  }
-  else {
+    /*
+     * The STK original 500 v1 protocol actually expects a_div = 1, but the
+     * v1.x FW of the STK500 kit has been superseded by v2 FW in the mid
+     * 2000s. Since optiboot, arduino as ISP and others assume a_div = 2,
+     * better use that. See https://github.com/avrdudes/avrdude/issues/967
+     */
+    a_div = 2;
+  } else {
     return -2;
   }
 
-  if ((m->op[AVR_OP_LOADPAGE_LO]) || (m->op[AVR_OP_READ_LO]))
-    a_div = 2;
-  else
-    a_div = 1;
-
   n = addr + n_bytes;
   for (; addr < n; addr += block_size) {
     // MIB510 uses fixed blocks size of 256 bytes
@@ -973,7 +972,7 @@ static int stk500_paged_load(PROGRAMMER * pgm, AVRPART * p, AVRMEM * m,
   }
     else {
       if (buf[0] != Resp_STK_OK) {
-        avrdude_message(MSG_INFO, "\n%s: stk500_paged_load(): (a) protocol error, "
+        avrdude_message(MSG_INFO, "\n%s: stk500_paged_load(): (b) protocol error, "
                         "expect=0x%02x, resp=0x%02x\n",
                         progname, Resp_STK_OK, buf[0]);
         return -5;
@@ -1037,7 +1036,8 @@ static int stk500_set_fosc(PROGRAMMER * pgm, double v)
   static unsigned ps[] = {
     1, 8, 32, 64, 128, 256, 1024
   };
-  int idx, rc;
+  size_t idx;
+  int rc;
 
   prescale = cmatch = 0;
   if (v > 0.0) {
@@ -1155,7 +1155,7 @@ static int stk500_getparm(PROGRAMMER * pgm, unsigned parm, unsigned * value)
     return -3;
   }
   else if (buf[0] != Resp_STK_OK) {
-    avrdude_message(MSG_INFO, "\n%s: stk500_getparm(): (a) protocol error, "
+    avrdude_message(MSG_INFO, "\n%s: stk500_getparm(): (b) protocol error, "
                     "expect=0x%02x, resp=0x%02x\n",
                     progname, Resp_STK_OK, buf[0]);
     return -3;
@@ -1214,9 +1214,9 @@ static int stk500_setparm(PROGRAMMER * pgm, unsigned parm, unsigned value)
     return -3;
   }
   else {
-    avrdude_message(MSG_INFO, "\n%s: stk500_setparm(): (a) protocol error, "
+    avrdude_message(MSG_INFO, "\n%s: stk500_setparm(): (b) protocol error, "
                     "expect=0x%02x, resp=0x%02x\n",
-                    progname, Resp_STK_INSYNC, buf[0]);
+                    progname, Resp_STK_OK, buf[0]);
     return -3;
   }
 }
diff --git a/src/stk500v2.c b/src/stk500v2.c
index 523d6539..a50fda4c 100644
--- a/src/stk500v2.c
+++ b/src/stk500v2.c
@@ -392,9 +392,7 @@ static int stk500v2_send_mk2(PROGRAMMER * pgm, unsigned char * data, size_t len)
 
 static unsigned short get_jtagisp_return_size(unsigned char cmd)
 {
-  int i;
-
-  for (i = 0; i < sizeof jtagispcmds / sizeof jtagispcmds[0]; i++)
+  for (size_t i = 0; i < sizeof jtagispcmds / sizeof jtagispcmds[0]; i++)
     if (jtagispcmds[i].cmd == cmd)
       return jtagispcmds[i].size;
 
@@ -481,7 +479,6 @@ static int stk500v2_jtag3_send(PROGRAMMER * pgm, unsigned char * data, size_t le
 static int stk500v2_send(PROGRAMMER * pgm, unsigned char * data, size_t len)
 {
   unsigned char buf[275 + 6];		// max MESSAGE_BODY of 275 bytes, 6 bytes overhead
-  int i;
 
   if (PDATA(pgm)->pgmtype == PGMTYPE_AVRISP_MKII ||
       PDATA(pgm)->pgmtype == PGMTYPE_STK600)
@@ -500,12 +497,13 @@ static int stk500v2_send(PROGRAMMER * pgm, unsigned char * data, size_t len)
 
   // calculate the XOR checksum
   buf[5+len] = 0;
-  for (i=0;i<5+len;i++)
+  for (size_t i=0; i<5+len; i++)
     buf[5+len] ^= buf[i];
 
   DEBUG("STK500V2: stk500v2_send(");
-  for (i=0;i<len+6;i++) DEBUG("0x%02x ",buf[i]);
-  DEBUG(", %d)\n",len+6);
+  for (size_t i=0; i<len+6; i++)
+    DEBUG("0x%02x ", buf[i]);
+  DEBUG(", %d)\n", (int) len+6);
 
   if (serial_send(&pgm->fd, buf, len+6) != 0) {
     avrdude_message(MSG_INFO, "%s: stk500_send(): failed to send command to serial port\n",progname);
@@ -551,9 +549,9 @@ static int stk500v2_jtagmkII_recv(PROGRAMMER * pgm, unsigned char *msg,
             progname);
     return -1;
   }
-  if (rv - 1 > maxsize) {
+  if ((size_t) rv - 1 > maxsize) {
     avrdude_message(MSG_INFO, "%s: stk500v2_jtagmkII_recv(): got %u bytes, have only room for %u bytes\n",
-                    progname, (unsigned)rv - 1, (unsigned)maxsize);
+                    progname, (unsigned) rv - 1, (unsigned) maxsize);
     rv = maxsize;
   }
   switch (jtagmsg[0]) {
@@ -597,9 +595,9 @@ static int stk500v2_jtag3_recv(PROGRAMMER * pgm, unsigned char *msg,
      implementation of JTAGICE3, as they always request a full 512
      octets from the ICE.  Thus, only complain at high verbose
      levels. */
-  if (rv - 1 > maxsize) {
+  if ((size_t) rv - 1 > maxsize) {
     avrdude_message(MSG_DEBUG, "%s: stk500v2_jtag3_recv(): got %u bytes, have only room for %u bytes\n",
-                      progname, (unsigned)rv - 1, (unsigned)maxsize);
+                      progname, (unsigned) rv - 1, (unsigned) maxsize);
     rv = maxsize;
   }
   if (jtagmsg[0] != SCOPE_AVR_ISP) {
@@ -814,13 +812,13 @@ retry:
 
 static int stk500v2_command(PROGRAMMER * pgm, unsigned char * buf,
                             size_t len, size_t maxlen) {
-  int i;
   int tries = 0;
   int status;
 
   DEBUG("STK500V2: stk500v2_command(");
-  for (i=0;i<len;i++) DEBUG("0x%02x ",buf[i]);
-  DEBUG(", %d)\n",len);
+  for (size_t i=0; i<len; i++)
+     DEBUG("0x%02x ",buf[i]);
+  DEBUG(", %d)\n", (int) len);
 
 retry:
   tries++;
@@ -991,6 +989,7 @@ static int stk500v2_chip_erase(PROGRAMMER * pgm, AVRPART * p)
   buf[0] = CMD_CHIP_ERASE_ISP;
   buf[1] = p->chip_erase_delay / 1000;
   buf[2] = 0;	// use delay (?)
+  memset(buf+3, 0, 4);
   avr_set_bits(p->op[AVR_OP_CHIP_ERASE], buf+3);
   result = stk500v2_command(pgm, buf, 7, sizeof(buf));
   usleep(p->chip_erase_delay);
@@ -1121,8 +1120,8 @@ retry:
   buf[5] = p->bytedelay;
   buf[6] = p->pollvalue;
   buf[7] = p->pollindex;
+  memset(buf+8, 0, 4);
   avr_set_bits(p->op[AVR_OP_PGM_ENABLE], buf+8);
-  buf[10] = buf[11] = 0;
 
   rv = stk500v2_command(pgm, buf, 12, sizeof(buf));
 
@@ -1957,12 +1956,12 @@ static int stk500isp_read_byte(PROGRAMMER * pgm, AVRPART * p, AVRMEM * mem,
     buf[0] = CMD_READ_SIGNATURE_ISP;
   }
 
-  memset(buf + 1, 0, 5);
   if ((op = mem->op[AVR_OP_READ]) == NULL) {
     avrdude_message(MSG_INFO, "%s: stk500isp_read_byte(): invalid operation AVR_OP_READ on %s memory\n",
                     progname, mem->desc);
     return -1;
   }
+  memset(buf+2, 0, 4);
   avr_set_bits(op, buf + 2);
   if ((pollidx = avr_get_output_index(op)) == -1) {
     avrdude_message(MSG_INFO, "%s: stk500isp_read_byte(): cannot determine pollidx to read %s memory\n",
@@ -2314,6 +2313,7 @@ static int stk500v2_paged_write(PROGRAMMER * pgm, AVRPART * p, AVRMEM * m,
               progname, p->desc);
       return -1;
     }
+    memset(cmds, 0, sizeof cmds);
     avr_set_bits(m->op[AVR_OP_LOADPAGE_LO], cmds);
     commandbuf[5] = cmds[0];
 
@@ -2322,6 +2322,8 @@ static int stk500v2_paged_write(PROGRAMMER * pgm, AVRPART * p, AVRMEM * m,
               progname, p->desc);
       return -1;
     }
+
+    memset(cmds, 0, sizeof cmds);
     avr_set_bits(m->op[AVR_OP_WRITEPAGE], cmds);
     commandbuf[6] = cmds[0];
 
@@ -2335,6 +2337,7 @@ static int stk500v2_paged_write(PROGRAMMER * pgm, AVRPART * p, AVRMEM * m,
               progname, p->desc);
       return -1;
     }
+    memset(cmds, 0, sizeof cmds);
     avr_set_bits(wop, cmds);
     commandbuf[5] = cmds[0];
     commandbuf[6] = 0;
@@ -2346,6 +2349,7 @@ static int stk500v2_paged_write(PROGRAMMER * pgm, AVRPART * p, AVRMEM * m,
             progname, p->desc);
     return -1;
   }
+  memset(cmds, 0, sizeof cmds);
   avr_set_bits(rop, cmds);
   commandbuf[7] = cmds[0];
 
@@ -2549,6 +2553,7 @@ static int stk500v2_paged_load(PROGRAMMER * pgm, AVRPART * p, AVRMEM * m,
             progname, p->desc);
     return -1;
   }
+  memset(cmds, 0, sizeof cmds);
   avr_set_bits(rop, cmds);
   commandbuf[3] = cmds[0];
 
@@ -2753,7 +2758,8 @@ static int stk500v2_set_fosc(PROGRAMMER * pgm, double v)
   static unsigned ps[] = {
     1, 8, 32, 64, 128, 256, 1024
   };
-  int idx, rc;
+  size_t idx;
+  int rc;
 
   prescale = cmatch = 0;
   if (v > 0.0) {
@@ -2774,7 +2780,7 @@ static int stk500v2_set_fosc(PROGRAMMER * pgm, double v)
       fosc = (unsigned)v;
 
     for (idx = 0; idx < sizeof(ps) / sizeof(ps[0]); idx++) {
-      if (fosc >= STK500V2_XTAL / (256 * ps[idx] * 2)) {
+      if ((unsigned) fosc >= STK500V2_XTAL / (256 * ps[idx] * 2)) {
         /* this prescaler value can handle our frequency */
         prescale = idx + 1;
         cmatch = (unsigned)(STK500V2_XTAL / (2 * fosc * ps[idx])) - 1;
@@ -2821,7 +2827,7 @@ static double avrispmkIIfreqs[] = {
 
 static int stk500v2_set_sck_period_mk2(PROGRAMMER * pgm, double v)
 {
-  int i;
+  size_t i;
 
   for (i = 0; i < sizeof(avrispmkIIfreqs) / sizeof(avrispmkIIfreqs[0]); i++) {
     if (1 / avrispmkIIfreqs[i] >= v)