Interfacing a slow device to a MCF5270 Microcontroller

Question

Hi,

I'm looking for ways to interface a slow device (an ADC - ~1.2MHz) to my mcf5270 microcontroller, which is a rather fast device (~150MHz). I am using ADC0817, and have tried the following: GPIO read off the data lines, memory mapped access (since the adc is uP compatible).

When mapped as a location in memory, I am using the maximum wait states (15ws, which is about 107ns). The mcf5270 does not support any further waiting without using their bus model (which the ADCs do not support).

Using GPIO, only junk values are read.

Note: The ADC is actually working, and other than reading data the interface to it is functional. I am using C/C++ and assembly.

In short, I am looking for suggestions for ways in which to try to read the data off the ADC0817.

Comments and responses are greatly appreciated.

Answer 1

I think memory mapped should work - normally I would wait for the interrupt from the ADC, then read the byte at the specified address. Since you get an interrupt, there is no wait state needed. Am I missing something? GPIO should also work. How do you know the ADC is working? Can you put a logic analyzer on the data & interrupt to prove the ADC is producing the correct output?

Answer 2

You could trigger the ADC through some GPIO, do an appropriate number of NOPs and then read the value. Also, you'd need to disable interrupts while doing this.

Answer 3

I think from what he's saying MMIO wont' work because he would need more than the maximum number of wait states on the bus to get the glue logic to respond - ie, the A/D won't set its bus pins soon enough for the uC to be happy. That's a problem sometimes.

But the GPIO approach should work. If junk values are being read I assume that you aren't following the timing diagram provided. You'll have to be able to wait certain amounts of time after you signal a read before the data is valid. Check out the datasheet for specifics.