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

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

bob@jfcl.com wrote:

> 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
> 

Thanks to all the people who offered suggestions.  In the end I
never did figure out the cause of the problem - it proved to be more
expedient to just use a fast cache SRAM (access time around 35ns)
together with a slow latch (74HC373 - tpd around 35ns!) to get a 70ns
memory system.

7C199 35ns 32Kx8 SRAMs are available from Jameco, qty 1, for less
than $2 each.  That's the same or less than what a 62256 costs.  The
7C199 is a power hog compared to the 62256 but that's not an issue for
this application.

Anyway, thanks again,
Bob Armstrong