Compiler/TM4C123GH6PM: Keil

Muhammad Usman Qadeer said:

What to write in ISR?

Just the ISR?   Everything else - "A-Ok?"   (I think not.)

PWM generator seems prime for such task.   You'll need to switch the (frequency) parameter to achieve (either) 100KHz or 50KHz w/in the PWM Frequency Set Command.

Your "1Kbps" rate means each bit is active for 1mS.   That 1mS is likely to result from your set-up of an MCU Timer which first "reads" your data - then loads the appropriate (frequency) value w/in the API Command previously mentioned.  Thus - your Timer's ISR causes a data read, data track (which bit is being processed), then loads the PWM Frequency parameter, and only then - outputs the PWM for 1mS.  (on a "bit by bit" basis)   (there may be a less efficient means to transfer data - but what you've described is surely a leading candidate...)

"How to do that?"   Good read of the MCU manual and the Peripheral Driver User Guide - and of course, "Use the API - NOT Direct Register!"   Examples abound both for Timer & PWM Set-Up & application - they appear under: Peripherals/Examples/etc.

Target device (not revealed) must be (highly) specialized to require such (involved & s l o w) data.