Allow loading an OSCCAL value from EEPROM

When an EEPROM address is given through OSCCAL_EEPROM_ADDR at
compiletime, a byte is read from that address on startup and written to
OSCCAL (unless it is 0xff, which should never be a meaningful
calibration value). This allows improving the accuracy of the UART
baudrate when running off the internal oscillator.

Enabling this option adds 30 bytes on the atmega328p build. This can
probably be reduced by inlining eeprom_read_byte.

Author: matthijskooijman

optiboot/bootloaders/optiboot/Makefile

@@ -190,8 +190,14 @@ ifdef SINGLESPEED
 SS_CMD = -DSINGLESPEED=1
 endif
 
+ifdef OSCCAL_EEPROM_ADDR
+OSCCAL_CMD = -DOSCCAL_EEPROM_ADDR=$(OSCCAL_EEPROM_ADDR)
+dummy = FORCE
+endif
+
 COMMON_OPTIONS = $(BAUD_RATE_CMD) $(LED_START_FLASHES_CMD) $(BIGBOOT_CMD)
 COMMON_OPTIONS += $(SOFT_UART_CMD) $(LED_DATA_FLASH_CMD) $(LED_CMD) $(SS_CMD)
+COMMON_OPTIONS += $(OSCCAL_CMD)
 
 #UART is handled separately and only passed for devices with more than one.
 ifdef UART

optiboot/bootloaders/optiboot/optiboot.c

@@ -128,6 +128,11 @@
 /* UART number (0..n) for devices with more than          */
 /* one hardware uart (644P, 1284P, etc)                   */
 /*                                                        */
+/* OSCCAL_EEPROM_ADDR                                     */
+/* On startup, load an oscillator calibration value from  */
+/* this address in EEPROM and write it to OSCCAL (unless  */
+/* it is 0xff).                                            */
+/*                                                        */
 /**********************************************************/
 
 /**********************************************************/
@@ -468,6 +473,13 @@ int main(void) {
   SP=RAMEND;  // This is done by hardware reset
 #endif
 
+#if defined(OSCCAL_EEPROM_ADDR)
+  // Load OSCCAL before app start, so the app does not have to
+  ch = eeprom_read_byte((uint8_t*)OSCCAL_EEPROM_ADDR);
+  if (ch != 0xff)
+    OSCCAL = ch;
+#endif
+
   /*
    * modified Adaboot no-wait mod.
    * Pass the reset reason to app.  Also, it appears that an Uno poweron

optiboot/bootloaders/optiboot/optiboot_atmega328.hex

@@ -1,33 +1,32 @@
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