V,

Since you mention that you are "scanning" a keyboard, that requires several i/o operations to sequence through each row of keys and then look at the columns within the selected row. All of this takes several IO operations. And since i2c is being used, each needed io operation is one or more i2c data byte sent in a serial fashion.

What I mean by nested is that the Scan routine most likely uses interrupts to recieve the i2c data. But, this process is started from within an interrupt. Even if interrupts are not used for i2c communication, doing several serial (i2c) data transactions within an interrupt is usually best avoided.

Another thing... You have not mentioned exactly how you have the keyboard connected to the port expander and how that will generate an interrupt. Do you enable all rows and wait for one of the columns to trigger? Then use use Scan to isolate the key?

<>< Lance.


V.Jayakumar wrote:
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Sir I know that the ISR cannot wait that long ,that is why it is vectored of to another place(#Int1Isr). Please be kind enough to explain to me the nesting within the interrupt here in this case