Prahlad,

As you are new to ICP so am I, but I can see that your hardware design needs revision

1) The way you have made 3V3 supply is not good. Output voltage will vary with load. Use some regulator (LDO type if you must feed in +5V)or revise your power supply design by using LM317 (if you can't get a fix output 3V3 regulator).

2) The voltage you get at ICP header is being passed through a transistor Q5, and you stated that you get 2.5V (with 3 diodes in PSU section). It seems that your transistor is not going into hard saturation. Try reducing the base resistor by half or use a transistor with higher beta.

3) How much load current is drawn by the device connected to ICP header? If you have a DMM ready, you can analyze where the voltage is getting dropped. While the device is connected to ICP, measure the voltage drop accross Q5 (emitter and collector). If it is equal (or almost equal) to the voltage loss i.e. 0.8V as you stated, the problem is Q5. Repeat the test without any device at ICP header and see if you get bit higher voltage at collector.

Don't use such type of power supplies with micros in any case and especially when programming them.