This exercise develops a serial I$/$O driver for the KL$05$Z board. The objective of this exercise is to implement interrupt$-$based serial communication with the KL$05$ UART using circular FIFO queues for receiving and transmitting serial data. An interrupt service routine $($ISR$)$ and serial character I$/$O driver are written and integrated with the circular FIFO queue operations test program from Lab Exercise Seven, and they are run on the KL$05$Z board.

Prelab Work

$1.$ Based on the specifications below and the notes you have taken in lecture, rewrite the following subroutines from prior lab exercises to work with receive and transmit queues rather than the UART data registers. You must write the subroutines so that no register other than output parameter register$($s$)$ and PSR has changed value after return.

$-$ GetChar: Dequeues a character from the receive queue, and returns it in R$0.$

$-$ PutChar: Enqueues the character from R$0$ to the transmit queue.

$2.$ Following the specifications in this exercise and the notes you have taken in lecture, write the ISR that handles UART$0$ transmit and receive interrupts: UARTO$\_$ISR. You must write the ISR so that no registers have changed value after return. $($Note: the Cortex$-$M$0+$ automatically preserves R$0-$R$3,$ R$12,$ LR$,$ PC$,$ and PSR for ISRs.$)$

$3.$ Write the change required in the KL$05$ vector table to "install" UARTO$\_$ISR.

$4.$ Write the subroutine Init$\_$UARTO$\_$IRQ to initialize the KL$05$ as discussed in class and presented in the class notes for interrupt$-$based serial I$/$O with UART$0$ through port B pins $1$ and $2$ using this format: one start bit, eight data bits, no parity, and one stop bit at $9600$ baud $-$ the same format and speed as previous lab exercises but with interrupts instead of polling. This subroutine should configure UART$0,$ should initialize the UART$0$ interrupt in the NVIC, and should initialize the receive and transmit queue management record structures $($RxQRecord and TxQRecord$)$ for $80-$character queue buffers $($RxQBuffer and TxQBuffer, respectively$)$ using InitQueue from Lab Exercise Seven. You must write the subroutine so that no registers have changed value after return. $($Suggestion: copy Init$\_$UARTO$\_$Polling that has been used since Lab Exercise Five, and modify it to use interrupts instead of polling, including changing UART$0$ initialization to support UARTO$\_$ISR and calling InitQueue from Lab Exercise Seven to initialize the receive and transmit queue management record structures.$)$