Problems using Dallas High Speed Micro (89C420) with 74F373 address latch

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bob 21 years ago

Hi,

I'm building an embedded controller that uses the Dallas 89C420 ultra high speed micro with an external 32K SRAM. There's no external program memory - I'm using the internal flash for that.

Right now it works just fine at 15Mhz with a 74HC373 address latch and a 70ns SRAM, however I want to run in clock doubled mode at 30Mhz. For this I figure I'll need to use a 74F373 latch instead of the HC part.

The trouble is that it doesn't work, even at 15Mhz 1X (i.e. not doubled) with the 74F part. Running a little memory diagnostic that I wrote, I get random SRAM errors that _don't_ occur with the 74HC address latch.

Are there any circuit changes that need to be made when using the

74F373 instead of the HC part? Am I going to need series damping resistors or pull up resistors on the F373s outputs?

There are lots of Dallas appnotes for the 320, 550 and 420 family that talk about replacing the 74HC latch with a 74F part, but I couldn't find one actual schematic where they did it! Thanks much, Bob Armstrong

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Jim Granville 21 years ago

The F373 does have TTL levels, whilst the HC373 has CMOS thresholds ?

You could try the newer Dallas 89C430/440/450, and also try a CMOS version of the F373 (as that is really a dinosaur), like the P74LVC573 from Philips (etc) ?

-jg

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bob 21 years ago

Thanks for the quick answers, guys.

Absolutely. Correct me if I'm wrong, but this shouldn't be a concern for CMOS outputs driving a TTL input (i.e. the 89C420 driving the

74F373) because the CMOS voltage swings are much wider than TTL requires. It _is_ an issue for TTL outputs driving CMOS inputs (i.e. the 74F373 driving the 62256 SRAM) because the TTL logic 1 level is pretty low by CMOS standards. BTW, Eric - that's the logic for adding pullups to the F373 outputs - to drag the TTL high levels up a little higher. I tried adding 4.7K pullups to the outputs of F373, and it did improve the output levels but it didn't fix the problem.

Sorry - I must have missed something - how would that help? I'd still need a faster address latch, right?

Thanks again, Bob Armstrong

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bob 21 years ago

Thanks for the quick answers, guys.

Absolutely. Correct me if I'm wrong, but this shouldn't be a concern for CMOS outputs driving a TTL input (i.e. the 89C420 driving the

74F373) because the CMOS voltage swings are much wider than TTL requires. It _is_ an issue for TTL outputs driving CMOS inputs (i.e. the 74F373 driving the 62256 SRAM) because the TTL logic 1 level is pretty low by CMOS standards. BTW, Eric - that's the logic for adding pullups to the F373 outputs - to drag the TTL high levels up a little higher. I tried adding 4.7K pullups to the outputs of F373, and it did improve the output levels but it didn't fix the problem.

Sorry - I must have missed something - how would that help? I'd still need a faster address latch, right?

Thanks again, Bob Armstrong

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Nicholas O. Lindan 21 years ago

What do you see with a 'scope: does the part meet setup/hold at the specified voltage levels?

Early 74F parts were notoriously noisy: you may need .01 || 0.1 uF decoupling caps just for that IC.

You don't mention what ram you are trying to drive with the F373.

Nicholas O. Lindan, Cleveland, Ohio Consulting Engineer: Electronics; Informatics; Photonics. Remove spaces etc. to reply: n o lindan at net com dot com psst.. want to buy an f-stop timer? nolindan.com/da/fstop/

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Jim Granville 21 years ago

It matters for noise margin.

They may have different noise characteristics. Newer devices and all...

Most 80C51 variants have a cut/paste warning from older devices, that ALE may have noise during multiple address bit transistions. I have never seen this in a real design, but on loaded devices it could happen.

You could try a series resistor PAD in the ALE to the F373, to slow the edge slightly, and shift the threshold - but I would avoid the F373....

-jg

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Mark Borgerson 21 years ago

I would suspect ground bounce as a potential problem. I haven't experienced it personally, but have had reports of such problems from another developer who was using similar latches on an IDE interface. In his case I think the solution involved some changes in PCB layout.

Mark Borgerson

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bob 21 years ago

Can't see any ground bounce on the scope, but that doesn't prove that it isn't there :-) I did try tacking a 1uF tantalum directly across the F373's power pins and that didn't help.

To answer some other questions people have posed - it is a PC board (not wire wrap) although only a two layer one so there is some inductance in the power supply leads depending on how good the routing is.

The SRAM chip in question is a 62256 (more exactly, a SEC KM62256CLP-7L).

It works fine with the HC373 latch, but not with a F373 or ACT373 part. I made no other changes except swapping the latch - the clock speed is still 15Mhz. I would have liked to try an AC373, but I don't have one handy.

Thanks again, Bob

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Steve Taylor 21 years ago

It wouldn't - the minimum impedance will have been at around 1Mhz or lower. You need to bypass the 1uF with a 10n ceramic - minimum impedance ~100Mhz.

Steve

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Paul Probert 21 years ago

If you look at the outputs of the 373 on a high speed scope (with a very short scope probe ground lead, please), I bet you see ringing. If so, you need to use some form of line flattening. The easiest is probably series 100 ohm resistors right at the outputs. This only works if all your loads are close together on the far end of the line.

Paul Probert

snipped-for-privacy@jfcl.com wrote:

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Problems using Dallas High Speed Micro (89C420) with 74F373 address latch

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