This is an initial draft of boot variables. Signed-off-by: Sean Cross <sean@xobs.io>
2.8 KiB
Fomu Boot Sequence
When Foboot starts, it must decide which mode to go into. The following modes are defined:
Mode Name | Short Name | Description |
---|---|---|
Foboot Rescue | FBU | Initial state -- rescue mode uses DFU |
Foboot Main | FBM | Presents a disk drive that makes it easy to load programs |
User Python Program | UPY | Uses a locally-stored Python interpreter to run Python code |
User RISC-V Program | URV | Uses the built-in RISC-V softcore to execute user programs |
User ICE40 Bitstream | UBS | Exits the Foboot RISC-V core and switches to the provided bitstream |
At start, FBU is loaded by the hardware. FBU will look for a special FBM signature at a given offset. If this signature exists, it will transfer execution to that program. Otherwise it will enter DFU Mode and await further communication.
Boot Flags
A series of Boot Fags determine how FBU and FBM hand over execution to the subsequent program.
Boot Flags are indicated by the magic number 0xb469075a
, followed by a 32-bit field, similar to the following C struct:
struct {
uint32_t magic; // 0xb469075a
uint32_t bitfield;
}
Bit Flag | Short Name | Description |
---|---|---|
00000001 | QPI_EN | Enable QPI mode on the SPI flash |
00000002 | DDR_EN | Enable DDR mode on the SPI flash |
00000004 | CFM_EN | Enable CFM mode on the SPI flash |
00000008 | FLASH_UNLOCK | Don't lock flash prior to running program |
00000010 | FLUSH_CACHE | Issue a "fence.i" prior to running user program |
00000020 | NO_USB_RESET | Don't reset the USB -- useful for debugging |
RAM boot
During development, it may be inconvenient to load a program onto SPI flash. Or maybe you want to run something that doesn't modify the contents of SPI. This is possible with RAM boot
.
If the DFU bootloader encounters the magic number 0x17ab0f23
within the first 56 bytes, then it will enable RAM boot mode. In this mode, the SPI flash won't be erased, and the program will be loaded to RAM.
Note that the value following the magic number indicates the offset where the program will be loaded to. This should be somewhere in RAM. 0x10001000
is a good value, and is guaranteed to not interfere with Foboot itself.
Magic Numbers
Value | Description |
---|---|
0xb469075a |
Denotes the start of boot flags bitfield |
0x17ab0f23 |
Loads the program into RAM instead of flash |
0x032bd37d |
Indicates a valid FBM image |