How to detect cold boot versus warm boot on an ARM processor?

Question

I'm looking for a way to determine whether an ARM processor is booting from a cold boot (i.e. initial power-on) versus a warm boot (i.e. reset assertion without actual power loss). Specifically I'm using an ARM968 core, will be making the determination using C or assembly, and I will use the determination so certain operations only run on the initial power-on and not on subsequent resets. In previous projects I've leveraged external circuitry (e.g. FPGA) to detect the different boot scenarios, but in this case I am limited to the ARM core.

Answer 1

You can initialize a global variable in RAM to a value that is unlikely during cold boot, and check for that during boot.

For microcontrollers normally the reset logic of the specific chip provides a status register, which indicates the source of the reset. I don't know if that exists for this bigger core, and whether you could use that.

Answer 2

It is likely to be difficult, and maybe you dont really mean just the core itself. The core should have gotten a reset, but the memory outside (but perhaps still within the chip) did not. if the memory is dram based then it may still get wiped on boot. I dont know of a generic one size fits all answer. both you and starblue have it though, you have to find some register somewhere that is not cleared on a reset, set that to something that is "likely" not to happen randomly on a power up. read it then set it. thinks like the fpga or pld that manage the reset logic at the board level (if any) are the best because on a power on reset they are reset as well, and on a warm reset they are the one that caused it and keep their state.

dig through the TRM for your core or through the register spec for the chip, and see if there are any registers whose reset state is undefined, one that you normally dont use and wont hurt the chip if you set it to something, and see what it powers up as, that is where I would start looking.

Answer 3

Check the docs for you specific chip ("ARM968" is not specific enough). There should be a register that describes the cause of reset. E.g. here's what LPC23xx has:

Reset Source Identification Register (RSIR - 0xE01FC180)

This register contains one bit for each source of Reset. Writing a 1 to any of these bits
clears the corresponding read-side bit to 0. The interactions among the four sources are
described below.

Bit Symbol Description
0 POR Assertion of the POR signal sets this bit, and clears all of the other bits in
this register. But if another Reset signal (e.g., External Reset) remains
asserted after the POR signal is negated, then its bit is set. This bit is not
affected by any of the other sources of Reset.
1 EXTR Assertion of the RESET signal sets this bit. This bit is cleared by POR,
but is not affected by WDT or BOD reset.
2 WDTR This bit is set when the Watchdog Timer times out and the WDTRESET
bit in the Watchdog Mode Register is 1. It is cleared by any of the other
sources of Reset.
3 BODR This bit is set when the 3.3 V power reaches a level below 2.6 V.
If the VDD(DCDC)(3V3) voltage dips from 3.3 V to 2.5 V and backs up, the
BODR bit will be set to 1.
If the VDD(DCDC)(3V3) voltage dips from 3.3 V to 2.5 V and continues to
decline to the level at which POR is asserted (nominally 1 V), the BODR
bit is cleared.
if the VDD(DCDC)(3V3) voltage rises continuously from below 1 V to a level
above 2.6 V, the BODR will be set to 1.
This bit is not affected by External Reset nor Watchdog Reset.
Note: Only in case when a reset occurs and the POR = 0, the BODR bit
indicates if the VDD(DCDC)(3V3) voltage was below 2.6 V or not.