Allow loading an OSCCAL value from PROGMEM

When the OSCCAL_PROGMEM variable is given at compiletime, a byte is
allocated in PROGMEM (flash) at a fixed location at the top of the flash
space (just below the version bytes, so the third byte from the top).

If this byte is overwritten with a calibration value (e.g. when
programming the bootloader), it will be automatically loaded on startup.
This allows improving the accuracy of the UART baudrate when running off
the internal oscillator, and prevents the actual program from having to
worry about calibration.

The default value of this byte is 0xff, which causes it to be ignored.

To fix the address of this variable, a dedicated .osccal section is used
for it, and the linker is passed some options to put it in the right
place (identical to what happens for the version variable).

Enabling this option adds 15 bytes on the atmega328p build.

Author: matthijskooijman

optiboot/bootloaders/optiboot/Makefile

@@ -59,7 +59,7 @@ export
 
 # defaults
 MCU_TARGET = atmega168
-LDSECTIONS  = -Wl,--section-start=.text=0x3e00 -Wl,--section-start=.version=0x3ffe
+LDSECTIONS  = -Wl,--section-start=.text=0x3e00 -Wl,--section-start=.osccal=0x3ffd -Wl,--section-start=.version=0x3ffe
 
 # Build environments
 # Start of some ugly makefile-isms to allow optiboot to be built
@@ -195,6 +195,11 @@ OSCCAL_CMD = -DOSCCAL_EEPROM_ADDR=$(OSCCAL_EEPROM_ADDR)
 dummy = FORCE
 endif
 
+ifdef OSCCAL_PROGMEM
+OSCCAL_CMD = -DOSCCAL_PROGMEM
+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)
@@ -246,7 +251,7 @@ virboot8: TARGET = atmega8
 virboot8: MCU_TARGET = atmega8
 virboot8: CFLAGS += $(COMMON_OPTIONS) '-DVIRTUAL_BOOT_PARTITION' '-Dsave_vect_num=EE_RDY_vect_num'
 virboot8: AVR_FREQ ?= 16000000L 
-virboot8: LDSECTIONS  = -Wl,--section-start=.text=0x1c00 -Wl,--section-start=.version=0x1ffe
+virboot8: LDSECTIONS  = -Wl,--section-start=.text=0x1c00 -Wl,--section-start=.osccal=0x1ffd -Wl,--section-start=.version=0x1ffe
 virboot8: $(PROGRAM)_virboot8.hex
 virboot8: $(PROGRAM)_virboot8.lst
 
@@ -255,7 +260,7 @@ virboot328: TARGET = atmega328
 virboot328: MCU_TARGET = atmega328p
 virboot328: CFLAGS += $(COMMON_OPTIONS) '-DVIRTUAL_BOOT_PARTITION'
 virboot328: AVR_FREQ ?= 16000000L
-virboot328: LDSECTIONS  = -Wl,--section-start=.text=0x7d80 -Wl,--section-start=.version=0x7ffe
+virboot328: LDSECTIONS  = -Wl,--section-start=.text=0x7d80 -Wl,--section-start=.osccal=0x7ffd -Wl,--section-start=.version=0x7ffe
 virboot328: $(PROGRAM)_virboot328.hex
 virboot328: $(PROGRAM)_virboot328.lst
 
@@ -303,7 +308,7 @@ atmega328: TARGET = atmega328
 atmega328: MCU_TARGET = atmega328p
 atmega328: CFLAGS += $(COMMON_OPTIONS)
 atmega328: AVR_FREQ ?= 16000000L
-atmega328: LDSECTIONS  = -Wl,--section-start=.text=0x7e00 -Wl,--section-start=.version=0x7ffe
+atmega328: LDSECTIONS  = -Wl,--section-start=.text=0x7e00 -Wl,--section-start=.osccal=0x7ffd -Wl,--section-start=.version=0x7ffe
 atmega328: $(PROGRAM)_atmega328.hex
 atmega328: $(PROGRAM)_atmega328.lst
 
@@ -322,7 +327,7 @@ atmega328_isp: isp
 atmega1280: MCU_TARGET = atmega1280
 atmega1280: CFLAGS += $(COMMON_OPTIONS) -DBIGBOOT $(UART_CMD)
 atmega1280: AVR_FREQ ?= 16000000L
-atmega1280: LDSECTIONS  = -Wl,--section-start=.text=0x1fc00  -Wl,--section-start=.version=0x1fffe
+atmega1280: LDSECTIONS  = -Wl,--section-start=.text=0x1fc00  -Wl,--section-start=.osccal=0x1fffd -Wl,--section-start=.version=0x1fffe
 atmega1280: $(PROGRAM)_atmega1280.hex
 atmega1280: $(PROGRAM)_atmega1280.lst
 
@@ -333,7 +338,7 @@ atmega8: TARGET = atmega8
 atmega8: MCU_TARGET = atmega8
 atmega8: CFLAGS += $(COMMON_OPTIONS)
 atmega8: AVR_FREQ ?= 16000000L 
-atmega8: LDSECTIONS  = -Wl,--section-start=.text=0x1e00 -Wl,--section-start=.version=0x1ffe -Wl,--gc-sections -Wl,--undefined=optiboot_version
+atmega8: LDSECTIONS  = -Wl,--section-start=.text=0x1e00 -Wl,--section-start=.osccal=0x1ffd -Wl,--section-start=.version=0x1ffe -Wl,--gc-sections -Wl,--undefined=optiboot_version
 atmega8: $(PROGRAM)_atmega8.hex
 atmega8: $(PROGRAM)_atmega8.lst
 
@@ -544,10 +549,10 @@ clean:
 	$(OBJDUMP) -h -S $< > $@
 
 %.hex: %.elf
-	$(OBJCOPY) -j .text -j .data -j .version --set-section-flags .version=alloc,load -O ihex $< $@
+	$(OBJCOPY) -j .text -j .data -j .version -j .osccal --set-section-flags .version=alloc,load --set-section-flags .osccal=alloc,load -O ihex $< $@
 
 %.srec: %.elf
-	$(OBJCOPY) -j .text -j .data -j .version --set-section-flags .version=alloc,load -O srec $< $@
+	$(OBJCOPY) -j .text -j .data -j .version -j .osccal -set-section-flags .version=alloc,load --set-section-flags .osccal=alloc,load -O srec $< $@
 
 %.bin: %.elf
-	$(OBJCOPY) -j .text -j .data -j .version --set-section-flags .version=alloc,load -O binary $< $@
+	$(OBJCOPY) -j .text -j .data -j .version -j .osccal --set-section-flags .version=alloc,load --set-section-flags .osccal=alloc,load -O binary $< $@

optiboot/bootloaders/optiboot/optiboot.c

@@ -133,6 +133,13 @@
 /* this address in EEPROM and write it to OSCCAL (unless  */
 /* it is 0xff).                                           */
 /*                                                        */
+/* OSCCAL_PROGMEM                                         */
+/* On startup, load an oscillator calibration value from  */
+/* the top of the flash memory (unless it is 0xff, which  */
+/* is the default). This byte is put into its own .osccal */
+/* section, so its address should be set through the      */
+/* linker.                                                */
+/*                                                        */
 /**********************************************************/
 
 /**********************************************************/
@@ -244,6 +251,11 @@
 unsigned const int __attribute__((section(".version"))) 
 optiboot_version = 256*(OPTIBOOT_MAJVER + OPTIBOOT_CUSTOMVER) + OPTIBOOT_MINVER;
 
+#if defined(OSCCAL_PROGMEM)
+unsigned const char __attribute__((section(".osccal")))
+optiboot_osccal = 0xff;
+#endif
+
 
 #include <inttypes.h>
 #include <avr/io.h>
@@ -480,6 +492,12 @@ int main(void) {
     OSCCAL = ch;
 #endif
 
+#if defined(OSCCAL_PROGMEM)
+  // Load OSCCAL before app start, so the app does not have to
+  ch = pgm_read_byte(&optiboot_osccal);
+  if (ch != 0xff)
+    OSCCAL = ch;
+#endif
   /*
    * modified Adaboot no-wait mod.
    * Pass the reset reason to app.  Also, it appears that an Uno poweron