[Amforth] AVR8: Mega 2560 Wiping Flash Memory

[Martin N. <amf...@mg...>]

I had problems with my Arduino Mega erasing parts of the flash memory;
sometimes the odd byte, sometimes the odd page.

I changed 3 things to the flashing code to make the problem go away:

1. Checked the SPMCSR:RWWSB bit. 
This is a crib from the Atmel example
(29.6.13 Simple Assembly Code Example for a Boot Loader).
From the docs: "When the RWWSB bit is set, the RWW section cannot be
accessed."

Cribbed code:
	; return to RWW section
	; verify that RWW section is safe to read
	Return:
	in temp1, SPMCSR
	sbrs temp1, RWWSB ; The RWW section is not ready yet
	ret
	; re-enable the RWW section
	ldi spmcrval, (1<<RWWSRE) | (1<<SPMEN)
	call Do_spm
	rjmp Return


2. Added a "Crash Trap" after NRWW_START_ADDR and immediately before the
flashing code. This performs a reset rather than bumbling on and
possibly overwriting some arbitrary FLASH.
I chose this snippet:
	nop   ; Some nops to synchronise.
	nop
	jmp_ PFA_COLD

Certainly a Forth in development can end up executing empty FLASH
memory. In the big Megas (store-i_big.asm) in particular, empty memory
leads inexorably to the flashing code. All this rather depends on the
function of M/C code instruction $FF, which seems undefined, so is
probably a noop.

3. Disabled interrupts much earlier.
Before any page is erased these should be switched off. Why?
The user may be writing to Page 0 or to the page containing "isr:".
Since time effectively stands still during flashing,
I issue a CLI at the earliest possible moment. The Atmel
example above says: "It is assumed that either the interrupt table is
moved to the Boot loader section or that the interrupts are disabled."
Under these circumstances the CLI in "dospm:" is no longer required.

Feel free to dispute any of these points; I'm all ears.

Remember: Aunty Atmel always knows best. She is omniscient here.

Source files:
amforth-6.9/avr8/words/store-i_big.asm
amforth-6.9/avr8/words/store-i_nrww.asm

-- 
Regards,

Martin Nicholas.

E-mail: rep...@mg... (Address will be valid throughout 2021).