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Old 13 January 2021, 12:02   #1
Ernst Blofeld
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CIA TOD Rollover

Editing post as my problem has now changed.

Can anyone explain why I get occasional silly values when reading the CIA B TOD clock?

When reading it at the start and end of a function, I occasionally get bad values, with the finish time being less than the end time, or even the finish time being zero sometimes.

More details in post 4 below.

Last edited by Ernst Blofeld; 13 January 2021 at 14:38.
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Old 13 January 2021, 12:51   #2
a/b
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Read once in this order: high (latch activated), middle, low byte (latch deactivated).
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Old 13 January 2021, 12:57   #3
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Quote:
Originally Posted by a/b View Post
Read once in this order: high (latch activated), middle, low byte (latch deactivated).
Ooooh... that explains why I couldn't find anything when I searched.

Thank you.
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Old 13 January 2021, 13:31   #4
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It's strange though, that I'm occasionally getting start times that are greater than my end times...

Code:
In Profiler_EndFunction: startTime 65532 > finishTime 0
In Profiler_EndFunction: startTime 192250 > finishTime 192000
In Profiler_EndFunction: startTime 258555 > finishTime 258304
In fact, how can I have a finish time of zero?

(edit - the low byte of all the finishTimes is zero, if that's a clue)

Code:
ULONG GetCurrentTime(void) {
    ULONG hi = ciab->ciatodhi;		// latch activated
    ULONG mid = ciab->ciatodmid;
    ULONG low = ciab->ciatodlow;	// latch deactivated

    return (hi << 8 | mid) << 8 | low;
}
ciab is
volatile struct CIA * ciab = CIAB;
where CIAB is
#define CIAB ((volatile struct CIA *) 0xbfd000UL)


The generated assembly looks sensible, to me:

Code:
00002c0a <GetCurrentTime>:
ULONG GetCurrentTime(void) {
    2c0a:	move.l d2,-(sp)
    ULONG hi = ciab->ciatodhi;		// latch activated
    2c0c:	move.b bfda00 <gcc8_c_support.c.62367fe7+0xbdadad>,d0
    ULONG mid = ciab->ciatodmid;
    2c12:	move.b bfd900 <gcc8_c_support.c.62367fe7+0xbdacad>,d2
    ULONG low = ciab->ciatodlow;	// latch deactivated
    2c18:	move.b bfd800 <gcc8_c_support.c.62367fe7+0xbdabad>,d1
    ULONG hi = ciab->ciatodhi;		// latch activated
    2c1e:	andi.l #255,d0
    return (hi << 8 | mid) << 8 | low;
    2c24:	lsl.l #8,d0
    2c26:	or.b d2,d0
    2c28:	lsl.l #8,d0
}
    2c2a:	or.b d1,d0
    2c2c:	move.l (sp)+,d2
    2c2e:	rts

Last edited by Ernst Blofeld; 13 January 2021 at 14:20.
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Old 13 January 2021, 15:57   #5
Toni Wilen
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Do you have OS running? TOD is reset periodically by system.
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Old 13 January 2021, 16:08   #6
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Quote:
Originally Posted by Toni Wilen View Post
Do you have OS running? TOD is reset periodically by system.
No, I've taken over the system, I believe correctly. The only interrupts that should be running are a copper one for my screen flipping and a ports one for the keyboard, which I believe only uses cia a.

Code:
void TakeSystem(void) {
	Forbid(); // forbid multitasking

	ActiView = GfxBase->ActiView; // save current view

	// Save current interrupts and DMA settings.
    systemADKCON = custom->adkconr;
    systemINTENA = custom->intenar;
    systemDMACON = custom->dmaconr;

	// The VBR is stored in a different place on 68010s and above.
    if ((SysBase->AttnFlags & AFF_68010)) {
		UWORD getVBR [] = {
			0x4e7a, 0x0801,     // movec.l vbr, d0
			0x4e73              // rte
		};

		VBR = (APTR *) Supervisor((void *) getVBR);
	}

    // Save system interrupt handers.
    for (int i = 0; i < 6; i++)
        systemInterruptHandlers[i] = VBR[25 + i];

	WaitForVerticalBlank(); WaitForVerticalBlank();

	custom->dmacon = DMAF_MASTER | DMAF_BLITHOG | DMAF_ALL; // clear all DMA channels

	// Set all colours to black.
	for (int a = 0; a <= 31; a++)
		custom->color[a] = 0x000;

	LoadView(NULL); // has side effect of enabling VERTB

	WaitTOF(); WaitTOF();

	WaitForVerticalBlank(); WaitForVerticalBlank();

	OwnBlitter();
	WaitForBlitter();

    custom->intena = INTF_INTEN | INTF_ALL; // disable all interrupts
    custom->intreq = INTF_INTEN | INTF_ALL; // clear any interrupts that were pending
}
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Old 13 January 2021, 17:18   #7
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At the moment I don't believe this is a problem with my code, because when the problem occurs the low byte of the is always zero. I could imagine something overwriting the variable on the stack, but only the lowest byte?

I've changed the code that detects the problem to the following:

Code:
    if (startTime > finishTime)
        * (WORD *) 0xdff180UL = 0xfff;
So that you get a white flash on the screen when it happens. On my set up this happens on around the 12th and 16th frames.

I've attached the executable in case anyone wants to have a look.
Attached Files
File Type: zip a.mingw.zip (28.7 KB, 132 views)
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Old 13 January 2021, 17:28   #8
Toni Wilen
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ALARM bit is set (CRB bit 7).
Reading TOD when ALARM bit is set won't latch the TOD but returns "live" TOD. (EDIT: which is undocumented feature)
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Old 13 January 2021, 17:41   #9
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I had just found this: https://eab.abime.net/showpost.php?p...8&postcount=14
But Toni already posted

Last edited by ross; 13 January 2021 at 19:43. Reason: removed a silly question :D
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Old 13 January 2021, 17:47   #10
Ernst Blofeld
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Quote:
Originally Posted by Toni Wilen View Post
ALARM bit is set (CRB bit 7).
Reading TOD when ALARM bit is set won't latch the TOD but returns "live" TOD. (EDIT: which is undocumented feature)
Thanks Toni.

I'm now clearing the ALARM bit,

Code:
    ciab->ciacrb = CIACRBF_ALARM;
in my init code, but not seeing any difference. Am I doing it right?
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Old 13 January 2021, 19:28   #11
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Like this?

Code:
    ciab->ciacrb &= ~CIACRBF_ALARM;
This seems to work, but it's hard to find good documentation and examples. This control register is not like the interrupt control registers, it should be read then a modified value written back to it?
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Old 13 January 2021, 19:52   #12
Toni Wilen
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Most CIA registers are normal read/write. Interrupt register is special.
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