How to make Microcontroller enter into ISP mode
The datasheet of controller has information on how to enter the ISP mode. 
Point 17.9 on page 112 explain: 

The boot loader can also be executed by forcing the device into ISP mode during a power-on sequence (see Figure 47). This is accomplished by powering up the device with the reset pin initially held low and holding the pin low for a fixed time after VDD rises to its normal operating value. This is followed by three, and only three, properly timed low-going pulses. Fewer or more than three pulses will result in the device not entering ISP mode. Timing specifications may be found in the data sheet for this device

Hope this helps.