adding blinker01 example, blinks the OK led.

README

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+
+Right, I know...Got my raspberry pi today.
+
+This repo serves as a collection of low level examples.  No operating
+system, embedded or low level embedded or deeply embedded, whatever
+your term is for this.
+
+From what we know so far there is a gpu on chip that boots off of I
+assume an on chip rom.  This goes to the sd card and does things.  it
+appears that the entry point for us as programmers is the kernel.img
+file, which appears to be the memory image copied into the ARM memory
+before releasing reset on the ARM processor.  The name of course because
+this is intended to be a Linux based educational computer, but it is
+just a file name, just a memory image.
+
+You will want to go here
+http://elinux.org/RPi_Hardware
+And get the datasheet for the part
+http://www.raspberrypi.org/wp-content/uploads/2012/02/BCM2835-ARM-Peripherals.pdf
+(might be an old link, find the one on the wiki page)
+And the schematic for the board
+http://www.raspberrypi.org/wp-content/uploads/2012/04/Raspberry-Pi-Schematics-R1.0.pdf
+(might be an old link, find the one on the wiki page)
+
+Ultimately I will make a uart based bootloader.  For now start with
+blinker01
+
+I dont normally use .data nor gcc libraries nor C libraries so you can
+build most if not all of my examples using a gcc cross compilerl.  Basically
+it doesnt matter if you use arm-none-linux-gnueabi or arm-none-eabi.
+what was formerly codesourcery.com still has a LITE version of their
+toolchain which is easy to come by, easy to install and well maybe not
+easy to use but you can use it.  Building your own toolchain from gnu
+sources (binutils and gcc) is fairly straight forward and at some point
+will create a script to do that for you.
+
+

blinker01/Makefile

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+
+ARMGNU ?= arm-none-eabi
+
+COPS = -Wall -O2 -nostdlib -nostartfiles -ffreestanding 
+
+all : kernel.img
+
+clean :
+	rm -f *.o
+	rm -f *.bin
+	rm -f *.elf
+	rm -f *.list
+	rm -f *.img
+
+novectors.o : novectors.s
+	$(ARMGNU)-as novectors.s -o novectors.o
+
+blinker01.o : blinker01.c
+	$(ARMGNU)-gcc $(COPS) -c blinker01.c -o blinker01.o
+
+kernel.img : memmap novectors.o blinker01.o 
+	$(ARMGNU)-ld novectors.o blinker01.o -T memmap -o blinker01.elf
+	$(ARMGNU)-objdump -D blinker01.elf > blinker01.list
+	$(ARMGNU)-objcopy blinker01.elf -O binary kernel.img
+
+
+
+
+
+

blinker01/README

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+
+See the top level README for information on where to find the
+schematic and programmers reference manual for the ARM processor
+on the raspberry pi.
+
+Based on some web searches the gpio is based at address 0x20200000 not
+0x7E200000 as shown in the documentation.  I have to go figure that out.
+
+Apparently what they have done is have the thing boot using the gpu
+in the part, dont bother, leave it be, they are not going to let us
+program it, not for a while at least.  There is I assume a gpu bootloader
+programmed in the chip, an OTP ROM based on something I read.  This
+searches for the sd card, and I assume loads another gpu bootloader
+from that, at some point, the arm memory is loaded from kernel.img
+on the sd card, then the ARM is released from reset.
+
+This simple example, sets up a small stack, not much to this program, and
+I have not read up on how much memory is actually in this space.  It
+then enables gpio16 as an output.  Then goes into a loop that sets
+the gpio, waits, resets it, waits, repeat.  gpio16 is shown on the
+schematic to connect to the OK led.  One of the bank of leds on the
+corner near the audio out and host usb ports.  The blink rate
+for me is a few blinks a second perhaps.  Future examples will get
+use a timer if there is one and narrow in on the clock setup, etc.
+
+Copy the kernel.img file to the sd card, overwriting whatever you had
+there.  You may wish to backup the one you had there before you started
+playing at this level.
+
+The beauty of this boot design is that it is not (easily) brickable,
+so far as we know.  With ignorance being bliss I will say it is not
+brickable.  Second we have no flash on the card we always run out of
+ram as far as we are concerned.  Making the board cheaper among other
+things.  The negative side to this is the development process is very
+painful.  It goes back to the GBA or NDS days, actually it was more
+painful then.  Here you unplug the rpi, pull the sd card out, put
+the sd card in an sd card reader, plug the sd card reader into the
+computer, wait or mount, copy kernel.img to the sd card, wait or sync,
+unmount the sd card, remove the sd card reader, remove the sd card
+place in rpi, plug in rpi.  And repeat with each new build.  I hope
+to get the uart figured out and a bootloader so that I/we only have
+to do that once, and from there on power off, power on, load the
+program over serial.  Not to dissimilar to the arduino experience.
+
+

blinker01/blinker01.c

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+
+//-------------------------------------------------------------------------
+//-------------------------------------------------------------------------
+
+extern void PUT32 ( unsigned int, unsigned int );
+extern unsigned int GET32 ( unsigned int );
+extern void dummy ( unsigned int );
+
+#define GPFSEL1 0x20200004
+#define GPSET0  0x2020001C
+#define GPCLR0  0x20200028
+
+//-------------------------------------------------------------------------
+int notmain ( void )
+{
+    unsigned int ra;
+
+    ra=GET32(GPFSEL1);
+    ra&=~(7<<18);
+    ra|=1<<18;
+    PUT32(GPFSEL1,ra);
+
+    while(1)
+    {
+        PUT32(GPSET0,1<<16);
+        for(ra=0;ra<0x100000;ra++) dummy(ra);
+        PUT32(GPCLR0,1<<16);
+        for(ra=0;ra<0x100000;ra++) dummy(ra);
+    }
+    return(0);
+}
+//-------------------------------------------------------------------------
+//-------------------------------------------------------------------------

blinker01/memmap

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+
+MEMORY
+{
+    ram : ORIGIN = 0x00000000, LENGTH = 0x1000
+}
+
+SECTIONS
+{
+    .text : { *(.text*) } > ram
+    .bss : { *(.bss*) } > ram
+}
+

blinker01/novectors.s

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+
+.globl _start
+_start:
+    b reset
+
+reset:
+    mov sp,#0x1000
+    bl notmain
+hang: b hang
+
+.globl PUT32
+PUT32:
+    str r1,[r0]
+    bx lr
+
+.globl GET32
+GET32:
+    ldr r0,[r0]
+    bx lr
+
+.globl dummy
+dummy:
+    bx lr