Add commands to read fuse settings, for SMC diagnostics (14 bytes), v…

…ersion 47.2.4 (#50)

* Add commands to read fuse settings, for SMC diagnostics (14 bytes)

New size: 7028 (max size 7680)

Reading fuse bits (and lock bits) is a hardware feature where you set
SPMCSR to (1 << RFLB) | (1 << SELFPRGEN), and then LPM within 4 clock
cycles.

The feature you read with LPM is decided by Z:
0: Low fuse
1: Lock bits
2: Extended fuse
3: High fuse

To keep firmware use at a minimum, assume that the numeric values of the
I2C commands are in the same order as the hardware feature, so that they
can share the code, and only do a simple subtract to separate between them.

X16 need to know which fuse settings to expect, and which fuse settings are
incorrect.

* Bump version to 47.2.4

README.md

@@ -26,6 +26,10 @@ If you want to update the SMC firmware, please read this [guide](doc/update-guid
 | 0x07      | Master read       | 1 byte            | Get keyboard keycode          | 
 | 0x08      | Master write      | 1 byte            | Echo                          |
 | 0x09      | Master write      | 1 byte            | Debug output                  |
+| 0x0a      | Master read       | 1 byte            | Get low fuse setting          |
+| 0x0b      | Master read       | 1 byte            | Get lock bits                 |
+| 0x0c      | Master read       | 1 byte            | Get extended fuse setting     |
+| 0x0d      | Master read       | 1 byte            | Get high fuse setting         |
 | 0x18      | Master read       | 1 byte            | Get keyboard command status   |
 | 0x19      | Master write      | 1 byte            | Send keyboard command         |
 | 0x1a      | Master write      | 2 bytes           | Send keyboard command         | 
@@ -79,6 +83,18 @@ Key codes are one byte. Bit 7 indicates if the key was pressed (0) or released (
 
 This command gets one keycode from the buffer. If the buffer is empty, it returns 0.
 
+## Get fuses and lock bits (0x0a..0x0d)
+
+These offsets returns the fuse settings and lock bits (low-lock-extended-high).
+
+Expected value:
+- Low: $F1
+- Lock: $FF
+- Extended: $FE
+- High: $D4
+
+The reason for this particular order, is size optimization in SMC FW.
+
 ## Get keyboard command status (0x18)
 
 This offset returns the status of the last host to keyboard command.

version.h

@@ -1,3 +1,3 @@
 #define version_major 47
 #define version_minor 2
-#define version_patch 3
+#define version_patch 4

x16-smc.ino

@@ -68,6 +68,10 @@
 #define I2C_CMD_ECHO                  0x08
 #define I2C_CMD_DBG_OUT               0x09
 #define I2C_CMD_GET_LONGPRESS         0x09
+#define I2C_CMD_GET_FUSE_LOW          0x0a
+#define I2C_CMD_GET_FUSE_LOCK         0x0b
+#define I2C_CMD_GET_FUSE_EXT          0x0c
+#define I2C_CMD_GET_FUSE_HIGH         0x0d
 #define I2C_CMD_GET_KBD_STATUS        0x18
 #define I2C_CMD_KBD_CMD1              0x19
 #define I2C_CMD_KBD_CMD2              0x1a
@@ -532,8 +536,21 @@ void I2C_Receive(int) {
   I2C_Data[0] = defaultRequest;
 }
 
+// readFuse:
+// Function must be called with interrupts disabled.
+// 0: Low
+// 1: Lock
+// 2: Extended
+// 3: High
+uint8_t readFuse(uint8_t address)
+{
+  eeprom_busy_wait();
+  return boot_lock_fuse_bits_get(address);
+}
+
 void I2C_Send() { 
-  switch (I2C_Data[0]) {
+  uint8_t request = I2C_Data[0];
+  switch (request) {
     case I2C_CMD_GET_KEYCODE_FAST:
       if (!sendKeyCode()) smcWire.clearBuffer();
       break;
@@ -602,6 +619,14 @@ void I2C_Send() {
     case I2C_CMD_SELF_PROGRAMMING_MODE: // Check if self programming mode is activated
       smcWire.write(selfProgrammingModeActive);
       break;
+
+    case I2C_CMD_GET_FUSE_LOW:
+    case I2C_CMD_GET_FUSE_LOCK:
+    case I2C_CMD_GET_FUSE_EXT:
+    case I2C_CMD_GET_FUSE_HIGH: // Read fuses
+      // Size optimization: Assume that numeric values of these commands are in this order, to fit with hardware
+      smcWire.write(readFuse(request - I2C_CMD_GET_FUSE_LOW));
+      break;
   }
 
   I2C_Data[0] = defaultRequest;