Generate uniq ID in cell H5 of sheet1 after putting the value in C6 of sheet1

Dear All,

I am new to this forum so please help me out i am facing problem in generating uniq Id. i used below formula to generate the uniq id but it refresh every time when i press enter in any cell so i want this value to be contant for once till the time it is not stored in 2nd sheet, and uniq id will be generate only after once put some value in cell C6 of sheet1 and if the cell value is null uniq id will be generated.

So pls help!

Code Used to generate uniq id:
=CHAR(RANDBETWEEN(65,90))&CHAR(RANDBETWEEN(65,90))&RANDBETWEEN(10,99)&CHAR(RANDBETWEEN(65,90))&CHAR(RANDBETWEEN(65,90))