--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/doc/Flash-boot-modes	Tue May 02 03:24:30 2017 +0000
@@ -0,0 +1,52 @@
+The Calypso chip includes an on-die boot ROM that allows the boot process to be
+interrupted and diverted by an external host sending some special characters
+into either of the two UARTs; this mechanism is what allows us to load code into
+RAM and to reload the flash on Calypso GSM devices without having to resort to
+JTAG or chip desoldering or other extreme measures.  In normal operation, when
+the boot path is NOT being diverted by an external serial download, the boot ROM
+transfers control to the regular firmware in the flash - but there are two
+different modes in which the flash fw image may be booted.
+
+In order for the flash fw image to be considered bootable by the Calypso boot
+ROM, the 32-bit word at flash address 0x2000 must equal either 0 or 1; if it
+equals any other value, the boot ROM will consider the flash fw image to be
+invalid (e.g., blank flash) and will wait forever for a serial download instead
+of proceeding with flash boot.  Depending on whether this word at 0x2000 equals
+0 or 1, the flash fw image will be booted in one of two very different ways;
+we shall call them flash boot mode 0 and flash boot mode 1, respectively.
+
+In flash boot mode 0 the following 32-bit word at flash address 0x2004 must
+contain the address of the flash fw image entry point (ARM/Thumb selection in
+the least-significant bit); the boot ROM will simply jump to this address with
+a BX instruction.  When the flash fw image is booted in this manner, the boot
+ROM is still mapped at address 0 and the first 8 KiB of flash are inaccessible
+except via the 0x03000000 alternate mapping, unless the firmware later changes
+the 0xFFFFFB10 register.  This boot mode is intended for flash fw images that
+use the interrupt and exception vectors in the ROM (branching to IRAM addresses
+0x80001C-0x800034) for their interrupt and exception handling.
+
+Flash boot mode 1 is different: instead of jumping directly to the flash fw
+image, the boot ROM copies a small piece of its code into IRAM and jumps to that
+code; the copied code disables the boot ROM via the 0xFFFFFB10 register (puts
+the external flash at address 0) and induces a processor reset through the
+watchdog timer.  It is not clear to us exactly what blocks are affected by the
+watchdog reset, but bits 9:8 of the 0xFFFFFB10 register are not reset, hence
+the ARM processor now boots from the reset vector in the flash as if the boot
+ROM weren't there - and the latter really is not there after having disabled
+itself.
+
+Flash boot mode 0 is only usable on Calypso C035 silicon (the "new" kind);
+while all commercial Calypso GSM devices targeted by FreeCalypso feature Calypso
+chips of the correct "new" kind, the people at TI who wrote and maintained their
+official firmware also had to work with older Calypso C05 chips featured on the
+early D-Sample and Leonardo boards.  The earlier boot ROM code version in those
+early Calypso chips also implements the two boot modes which we call mode 0 and
+mode 1, but its implementation of mode 0 is broken and unusable, therefore TI's
+firmware people only used flash boot mode 1.  On the other hand, newer firmware
+designs made for current rather than historical hardware will probably find
+mode 0 to be cleaner, more intuitive and more convenient.
+
+All TI official firmwares use flash boot mode 1, our FreeCalypso Magnetite
+firmware does likewise, being a direct derivative of TI's TCS211 fw, but our
+FC Citrine firmware uses flash boot mode 0, as that part of the Citrine fw is
+our own original design.