$1.$ a$)$ Write a function in C that uses SysTick delay for $1$ millisecond. Assume that the SysTick is already initialized with NVIC$\_$ST$\_$RELOAD$=0$x$00$FFFFFF; NVIC$\_$ST$\_$CTRL$=0$x$05$; Assume that the clock is running at $50$ MHz $,$ such that each bus cycle is $20$ ns $.$

b$)$ Write a C function code that initializes the SysTick to interrupt every $100$ us$.$ In this initialization, in addition to enabling and arming SysTick interrupts you should also enable interrupts in the processor. SysTick ISR should run at priority level $1.$ Assume the bus clock is $80$ MHz $.$ You do not need to write the SysTick ISR.