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Nov 20, 2009, 9:42pm

Petscii Forums "PETSCII.COM" :: General :: DTVHacking :: nNMI (Non-Maskable Interrupt) found

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Topic: nNMI (Non-Maskable Interrupt) found (Read 578 times)

gmoon
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nNMI (Non-Maskable Interrupt) found
« Thread Started on May 22, 2006, 4:32pm »

I've I.D.ed nNMI (Hummer again, sorry, no have C64DTV)

nNMI (Non-Maskable Interrupt) line is trace leading from ASIC to left side of R46 (beside R48). Since nNMI line is right next to nIRQ (refer to schematic), this was easy to find.

It's bi-directional, negative-assert, just like nIRQ.

It behaves as expected--unlike nIRQ, nNMI is 'edge' triggered, and only one interrupt is triggered when pulling line LO, no matter how long it is held LO. nIRQ, on the other hand, will keep triggering a maskable IRQ over and over, as long as it's LO.

[image]

Tested by--

OUTPUT:
Hold digital probe on trace (left side of resistor) then press "Pause/Break" key (corresponds to "RESTORE" on c64, one of the ways an nNMI is triggered.) This pulls the line LO, probe beeps once. For each keypress, a single beep.

INPUT:
Ground the trace while holding "ESC" key down. This initiates a "Warm Start", exactly like Holding "STOP" and hitting "RESTORE" on a c64. (The standard nNMI routine checks to see if "STOP" key is pressed.)

(A more definitive test would have involved changing the nNMI vector and then testing, but I think this is evidence enough.) Bogarting David's pic, again.

« Last Edit: May 22, 2006, 4:34pm by gmoon »

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