I'm working on a custom board with a i.MX RT1062 processor and NOR
flash connected to LPSPI3. I need to be able to boot this board from
NOR flash. Section of the reference manual 8.6.8 says this is supported
by the ROM.
However, even after burning the LPSPI_PORT_SEL fuses (0x6D0[26:25]) as
0b10, there's no activity on the LPSPI3 signals after a power on
reset. E.g., PCS0 and SCK remain at 0V and never change while the ROM
code is running. I've also burned BT_FUSE_SEL (0x460) with no
Examining SW_MUX_CTL for GPIO_AD_B0_00 in a debugger just after POR,
shows that pin (and the others for LPSPI3) is configured for
MUX_MODE=ALT5 (i.e., regular GPIO). If the ROM tried to use LPSPI3,
I'd expect these pins to be configured as ALT7.
The NOR flash works just fine when we load the application directly
into OCRAM with a debugger.
The reference manual is not particularly clear on the configuration
for booting from LPSPI. Table 8-10 says that BOOT_CFG[7:4] = 0b0000
is supposed to mean "Serial NOR device selected as boot
device". However, Table 8-9 specifically lists FlexSPI (not LPSPI) as
the interface for 0b0000.
So my question is, what specifically needs to happen in order for the
ROM to load the first 4k of data from LPSPI3 after a power on reset?