External EPROM code error

DrWhoF · Dec 6, 2005

External EPROM code

If I run code only from external EPROM everything runs OK (87C52).
As soon as I burn code to internal OTP and leave look-up tables in external EPROM (outside internal memory area), so they can be changed without changing the CPU code, nothing works anymore.
What is the reason?
Thanks.
DrWho

IanP · Dec 6, 2005

Re: External EPROM code

If the code is executed correctly from external EPROM then the problem most likely lays in the hardware ..
My guess is that you may have problem with pullups on Port 0 ..
Try to connect 10kΩ pullups on this Port and see how it goes ..

Regards,
IanP

Port 0 differs in that its internal pullups are not active during normal port operation. The pullup FET in the P0 output driver (see Figure 4) is used only when the port is emitting 1s during external memory accesses. Otherwise the pullup FET is off. Consequently P0 lines that are being used as output port lines are open drain. Writing a 1 to the bit latch leaves both output FETs off, so the pin floats. In that condition it can be used as a high-impedance input.

wek · Dec 6, 2005

External EPROM code

Did you set the security fuses?
Some derivatives prevent external code memory access when all the fuses are on.
Just a guess.

wek

DrWhoF · Dec 7, 2005

Re: External EPROM code

I tried with the security bits but it didn't make any difference.
As soon as I fitted 10kΩ pullups on port 0 circuit started to work again.
Thanks very, very much.
How did you know that?
DrWho

IanP · Dec 7, 2005

Re: External EPROM code

Just a guess ..

Regards,
IanP

External EPROM code error

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