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External ROM read problem

I have a circuit using a DS89C450 micro and an external 1m(128kx8)ROM. I am using P1.0 as an upper address bit which is low for this test. I have /CE tied low and use /OE derived from p3.7(/rd) to read from the ROM. When I do a movx a,@dptr /OE does go low and about 10ns later the data becomes valid but goes away after 30ns. Shouldn't the data be valid throughout the entire /OE ? When I send the data I received to the serial port, it was 00h and not what I saw for the 30ns but what was after the 30ns. I have msg'd the manufacturers tech support also. Any ideas?

Reply to
Ed
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I'm not familiar with the part you are using, but the following should still hold true.

The address should be valid for the min setup time of the ROM before the data will be valid. It shouldn't matter that the /OE is on during that time, but it needs to be valid before the processor latches the value. That means the data may not be valid during the entire period of /OE, but it may be. Depends on how your decoding and data latching occurs. What speed are you trying to access the ROM at and is it within the ability of the ROM to work at that speed?

Jim

Reply to
James Beck

P0 must have pull-ups.....

Reply to
martin.shoebridge

Make sure that you are latching the lower 8 bits of address (AD[0..7] on P0) with /ALE, the ROM needs a steady 17-bit address for the entire Read Cycle.

Reply to
Barbarian

Problem solved. I missed the pull-ups on P0 Thanks

Reply to
Ed

Excellent, just as I suggested. Many peeps get caught out by this.I have done lots of designs using the 450. Tricky little piece of silicon though and very 'buggy' but very powerful. The boot loader has to be one of the best I have ever used.

Reply to
martin.shoebridge

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