How to programmatically set cell value in DataGridView?

I have a DataGridView. Some of the cells receive their data from a serial port: I want to shove the data into the cell, and have it update the underlying bound object.

I'm trying something like this:

SetValueFromSerial (decimal newValue)
{
    dataGridView.CurrentCell.Value = newValue;
}

using a string doesn't help:

    dataGridView.CurrentCell.Value = newValue.ToString ();

In both cases, I don't see anything in the grid, and the underlying value is unchanged.

I did Google and search here, but I didn't find anything. (I may have missed something, perhaps something obvious, but I'm not utterly lazy.)