Page 367 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

344 Chapter 11 Input/ Output

that begins outputting Vref = 0, increments Vref until it just exceeds Vin, and returns
the byte it last output in PORTA. This is called a ramp converter.

19. An A-to-D converter is connected as in Problem 18. Write the shortest subroutine
SUCCESS that begins outputting Vref = 2.5 and recursively determines whether Vin is
in the upper or lower half of the range, reducing the range by one half each time it
determines which half of the range Vin is in. SUCCESS returns the final value output
on PORTA. This divide-and-conquer algorithm is like binary number division; it is called
successive approximation conversion.

20. For the compiled C procedure in Figure 11.18, encircle and label the following C
statements and assembly-language program segments: (1) declaration of global
variables; (2) declaration of I/O ports; (3) declaration, allocation, and deallocation of
local variables; (4) the initialization ritual; (5) for loop control statements to output
str; (6) while loop control statements to input until a carriage return; (7) gadfly loops;
(8) input operation; and (9) output operation. Next to each circle, write the label shown
above in italics or a number enclosed in parentheses () to identify which part the code
corresponds to. If one of the above appears in two parts, circle each part.

21. For the compiled C procedure in Figure 11.19, encircle and label the following C
and assembler program segments: (1) declaration of global variables; (2) declaration of
I/O ports; (3) declaration of local variables; (4) the initialization ritual; (5) the loop
control statements to try each value from 1 to 128; (6) the gadfly loop; (7) the input
operation; (8) the output operation; (9) calculation and collection of results; and (10)
an infinite loop to stop the program. Next to each circle, write the label shown above
in italics or a number enclosed in parentheses () to identify which part the code
corresponds to.

* volatile int SClBD@OxC8;volatile char SClCR@OxCB,SClSR@OxCC,SClDR@QxCF;
SClBDi EQU $C8
SC1CR: EQU $CB
SC1SR: EQU $CC
SC1DR: EQU $CF
ORG $800

* char j; const char str[10] = "Hi ThereXr";
j: DS.B 1
str: DC.B "Hi ThereXr"

* void put(char d) { while( ( SC1SR & 0x80 ) == 0) ; SC1DR = d; }

put: BRCLR SC1SR,$80,put
STAA SC1DR
RTS

* char get{) { while( ( SC1SR & 0x20 ) == 0) ; return SC1DR; }