pp so that a modular approach to circuit design may be adopted the space occupied by the io and protection circuitry is at the heart of the present embodiment as shown in fig4 a b and c in fig4 a a known pad and io protection circuit arrangement is shown the arrangement comprises io pads 24 which connect with respective divisible parts of the io interface known as io cells 26 which provide an interface to core circuitry not shown the io cells are io library cells which can be selected and implemented from a library of such cells the pad pitch u201c pp u201d defines the width of each io cell 26 in this case 40 u03bcm the pad height u201c ph u201d is the height of the io circuitry and hence also the height allocated to electrostatic discharge esd protection circuitry 28 and here is 400 u03bcm supply vdd and ground vss pads 23 are provided for the io circuitry and communicate therewith via vdd vss connections and esd circuitry 28 which protects the io circuit against voltage inversion or overload note that the pads shown as vdd or vss supply or ground are interchangeable in position the esd circuitry includes connections to provide supply vdd.