I have another question.

I have a board in for repair, actually several. All have the same issue. As you know I work mainly digital.

We have an 8051 from DS its an 89c450. and its has no SPI but has a software one.. I do not have the source so the software isn't viewable.

We have a loadcell connected to an amp then into an SPI MCP3201 12bit ADC.. The SDI in goes up to the MCU but via a schmitt hex buffer 74hc14 On the one side of the hex buffer is a weak pullup BUT!! either the MCP3201 SDI pin or the HEX input is shorted to ground.. Obviously some contention but I fail to see why..

I have asked them to place a 270 ohm resistor in the line but I cant see why it fails.
Any Ideas

schematic ?

Not mine so I can't show this But mcp3201 -> 74hc14 all digital signals 1&2 clk, 3&4 dout 5&6 CS.

From a practical point of view, outputs are far more likely to go S/C than inputs, so I'd suspect the MCP3201 has gone duff.

Could there possibly be large spikes coming through it's input?.

Maybe... I just found out that it is the same customer.. I suspect an external spike as the software SPI means the MCU will be floating and the input of the hex buffer is floating ( pulled up with a 10k ) so I cannot see any way other the external influence..

They will be handling the boards without protection... Just seems strange its the same track!!

I wonder if they are plugging in the ribbon cable wrong?

Ian Rogers said:

Maybe... I just found out that it is the same customer.. I suspect an external spike as the software SPI means the MCU will be floating and the input of the hex buffer is floating ( pulled up with a 10k ) so I cannot see any way other the external influence..

They will be handling the boards without protection... Just seems strange its the same track!!

I wonder if they are plugging in the ribbon cable wrong?

Click to expand...

If you are using FPC as ribbon cables, an upside down ribbon shouldn't be a problem if you have connectors that are either top OR bottom contacts only. You get into trouble if you have connectors with top AND bottom contacts.

Check if the 74HC14 input pin (connected to SDI) is floating or properly driven. If it’s floating, it can cause unpredictable behavior.