Homework seven
EECS 203
Due 30 May
Prepared by Robert Dick. Zero-point questions were prepared in collaboration with
teaching assistants.
When you can verify an answer, do so. I encourage you to use a PIC assembler and simulator to help
you verify your answers.
Show your work.
1. Consider the following sequence of PIC16C74A directives/pseudo-operations and instructions
BANKSEL TRISA
clrf TRISA
comf TRISA, F
(a) (5 pts.) Which of these are directives/pseudo-operations and which are instructions?
(b) (5 pts.) Use a sentence to describe the overall effect of this instruction sequence.
(c) (5 pts.) Assuming the output of a driver is connected to one of the pins on port a, why is this
sequence of instructions potentially dangerous?
(d) (5 pts.) If your processor is clocked at 3MHz, what is the maximum duration of the dangerous
condition? Please refer to the Architectural Overview section in the PIC16C74A datasheet.
(e) (5 pts.) Write an instruction sequence that will safely accomplish the same thing as these
potentially dangerous instructions.
(f) (5 pts.) Rewrite your answer to question 1e, replacing all directives with genuine sequences of
instructions. Please refer to the Status Register documentation in the PIC16C74A datasheet.
(g) (5 pts.) Write the binary 14-bit opcode (including register specifiers) for each line of your answer
to question 1f. Refer to the Table 15.2 in the PIC16C74A datasheet.
2. Consider the following subroutine
LOW_TICK equ 0x20
HIGH_TICK equ 0x21
delay_loop:
; Wait for 30 ms to make sure switch bouncing is finished
; 2 MHz clock, about 2 us per normal instruction, 4 us per taken loop
movlw d’30’
movwf HIGH_TICK
LONG_WAIT_LOOP
; Wait for approx. 1 ms
SHORT_WAIT
movlw d’100’
movwf LOW_TICK
; Loop body takes 10 us
SHORT_WAIT_LOOP
nop
1
