UPDI: Only write fuses whose value needs to be changed. This fixes an… (

#11) * UPDI: Only write fuses whose value needs to be changed. This fixes an issue where writing the fuses locks the device. It seems writing any value to lockbits (including "valid key" 0xC5") will lock the device, and the library always wrote all fuses including lockbits. Not writing fuses whose values have not changed fixes this. * Fix formatting Co-authored-by: Peter Kooiman <[email protected]>
adafruit · Jan 26, 2022 · dcaaa13 · dcaaa13
1 parent 4c0a545
commit dcaaa13
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Showing 2 changed files with 22 additions and 3 deletions.
diff --git a/Adafruit_UPDIProg.cpp b/Adafruit_UPDIProg.cpp
@@ -485,6 +485,7 @@ bool Adafruit_AVRProg::updi_run_tasks(uint16_t tasks, uint8_t *data,
   uint8_t saved_fuses[AVR_NUM_FUSES];
   if (tasks & UPDI_TASK_WRITE_FUSES) {
     // we need to preserve the fuses through the device setup process
+    // g_updi.fuses contains the new fuse values
     for (uint8_t i = 0; i < AVR_NUM_FUSES; i++)
       saved_fuses[i] = g_updi.fuses[i];
   }
@@ -570,8 +571,10 @@ bool Adafruit_AVRProg::updi_run_tasks(uint16_t tasks, uint8_t *data,
       // info was previously fetched for all operations
     }
 
-    // save fuses into updi array
-    if (tasks & UPDI_TASK_READ_FUSES) {
+    // save fuses into updi array, also read before writing the fuses
+    // to determine which ones actually need to be changed
+    // New fuse values have been saved in saved_fuses on function entry
+    if ((tasks & UPDI_TASK_READ_FUSES) || (tasks & UPDI_TASK_WRITE_FUSES)) {
       Serial.printf("Reading fuses\n");
 
       for (uint8_t i = 0; i < AVR_NUM_FUSES; i++) {
@@ -584,8 +587,18 @@ bool Adafruit_AVRProg::updi_run_tasks(uint16_t tasks, uint8_t *data,
     if (tasks & UPDI_TASK_WRITE_FUSES) {
       Serial.printf("Writing fuses\n");
 
+      // Only write fuses whose value needs to be changed
+      // Especially important for lockbits, as writing any value
+      // to lockbits (including 'valid key' 0xC5) seems to lock the device
       for (uint8_t i = 0; i < AVR_NUM_FUSES; i++) {
-        updi_write_fuse(i, saved_fuses[i]);
+        if (saved_fuses[i] != g_updi.fuses[i]) {
+          DEBUG_FUSES("Write fuse %d: 0x%02X -> 0x%02X\n", i, g_updi.fuses[i],
+                      saved_fuses[i]);
+          updi_write_fuse(i, saved_fuses[i]);
+        } else {
+          DEBUG_FUSES("Skip fuse %d: 0x%02X -> 0x%02X\n", i, g_updi.fuses[i],
+                      saved_fuses[i]);
+        }
       }
     }
 
@@ -1033,6 +1046,9 @@ uint8_t Adafruit_AVRProg::updi_read_fuse(uint8_t fuse) {
 bool Adafruit_AVRProg::updi_write_fuse(uint8_t fuse, uint8_t value) {
   uint8_t data;
 
+  DEBUG_FUSES("updi_write_fuse: fuse %d value 0x%02X\n", fuse, value);
+  // Serial.print("Write fuse "); Serial.println(fuse, DEC);
+
   if (!updi_is_prog_mode()) {
     DEBUG_VERBOSE("in updi_write_fuse() error: not in prog mode\n");
     return false;

diff --git a/Adafruit_UPDIProg.h b/Adafruit_UPDIProg.h
@@ -54,6 +54,9 @@ SOFTWARE.
 //#define DEBUG_PHYSICAL(fmt, ...)	Serial.printf(fmt, ##__VA_ARGS__)
 #define DEBUG_PHYSICAL(fmt, ...) {}
 
+#define DEBUG_FUSES(fmt, ...)	Serial.printf(fmt, ##__VA_ARGS__)
+//#define DEBUG_FUSES(fmt, ...) {}
+
 // clang-format on
 
 #define UPDI_BREAK 0x00