diff --git a/src/stk500.c b/src/stk500.c
index b3ed3980..084bbec8 100644
--- a/src/stk500.c
+++ b/src/stk500.c
@@ -1034,7 +1034,8 @@ 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) {
diff --git a/src/stk500v2.c b/src/stk500v2.c
index b0ccaad2..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++;
@@ -2760,7 +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) {
@@ -2781,7 +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;
@@ -2828,7 +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)