LCD just show grey square

[winson]

Finally the LCD is WORKING!

The problem is in the power supply. The 5V Vdd supply voltage drop to 4.33V when it is connect to the LCD. I was using a battery for the LCD, due to high supply current of the 9V battery, thus have to put a current limiting resistor to limit the current that goes into the LCD, And i think because of the LCD consist of certain amount of internal resistance(from calculation it is around 23.23K ohms), this resistance will form a voltage divider with my current limiting resistor and make the supply voltage to the LCD drop to 4.33V. Normally, for LCD it's Vdd range is from 4.5V to 5.5V in order for it to work correctly.

After adjust the value of the current limiting resistor to get back a Vdd which is around 4.6V then the LCD work nicely. Besides, for the Vo of the LCD i was directly connect it to ground to get maximum contrast, this is to avoid the variable resistor(pot) that use to control the contrast from disturbing the voltage value of the Vdd.

For the send_command() and send_data() function i was modify the code to first load the data to the Port D then toggle the E signal from low to high then goes back to low again. I think it also work by set the E signal to high first then load data to Port D and after that set E to low, but since the LCD is working then i will follow the modified code.

Thanks everybody for giving all the useful information!

[PCM programmer]

Normally a voltage regulator I.C. is used to avoid this problem.
A voltage regulator will maintain +5.0v within its specified maximum
current limit. The most commonly known type is called LM7805.