There is another possible solution to make a heart beat counter using 8051 and that is connect your detector to the interrupt input and then to count the number of interrupts made by each heart beat.You will have to write a code for each interrupt made by your detector. You can also modify your programme to adjust the baud rate( using inbuilt UART) in such a manner that the time taken to calculate the heart beat per minute is calculated within 10-15 seconds. this will save your time and also it will increase the efficiency and repeatability of your project. This will also ensure that the data is in your 8051 and so you can also modify it in any way(if you need to or your approach to prograame so written is different then I know of).