EECS 373 Fall 2018 Homework #3

Transcription

1 EECS 373 Fall 2018 Homework #3 Answers 1) Loaders, Linkers and Executables a) In straightforward English, explain the role of a linker. [7 points] A linker receives object files as input and must emit an executable, library, or another object file. This involves cross-referencing symbol tables between object files as well as laying out the sections (data, text, etc.) of a program in memory. b) Consider the linker script at Briefly explain why the label eoa is at a higher address than the label stop even though the label eoa appears first. (Though largely irrelevant to the question, be aware this appears to be ARM not Thumb assembly). [8 points] The linker script specified that the data section come after the text section. Since eoa is in the data section (which starts at location 0x400, it is at a higher (larger) address than any label in the text section (which starts at location 0x000), including stop.

2 2) Write rewrite the function A in UAL assembly which does the same thing as the following C code. You should assume print is some ABI compliant function which takes a single integer argument and does _something_ with a (probably prints it somewhere eh?) and B is also an ABI compliant function. (Note, one point of the ABI is to be able to mix C and assembly like this, the linker will make it all work!) [15 points] void main() { int a=4,b=3; b=a(a,b); print(b); int A(int x, int y) { int a; a=b(x+y); print(a); There are a number of ways to do this, here is params: r0 = x, r1 = y A: push {lr, r4, r5, r6 mov r4, r0 mov r5, r1 add r0, r0, r0 = x + y bl a = B(r0) mov r6, r0 bl print(r0) add r0, r6, r0 = a + x sub r0, r0, r0 -= y pop {lr, r4, r5, r6 bx lr

3 3) Consider the basic example from lecture 3 (starting slide 23). a) Write a short UAL assembly program which every time the button is pressed toggles the light (so that if it s off pressing the button turns it back on and if it s on pressing the button turns it off. You can assume the switch is debounced. Be sure that pressing the button doesn t cause the light to quickly flicker rather each button press (off to on) should cause the light to toggle. [10 this code assumes the button is active-high after debounce mov r3, read button address mov r2, write LED address mov r0, LED state mov r4, button state loop: ldr r1, get button state cmp r4, check for rising edge. Must have been low and bne now be high cmp r1, check for button press beq same eor r0, r0, toggle LED str r0, [r2] same: mov r4, save button state b loop b) The same as part a, but in C this time. [10 points] #include <stdint.h> #define BUTTON_ADDR 0x05 #define LED_ADDR 0x04 int main() { volatile uint8_t *button_ptr = (uint8_t *) BUTTON_ADDR ; volatile uint8_t *led_ptr = (uint8_t *) LED_ADDR; uint8_t prev_button_down = 0; uint8_t led_on = 0; while (1) { // this code assumes the button is active high after debounce uint8_t button_down = *button_ptr; if (!prev_button_down && button_down) { led_on =!led_on; *led_ptr = led_on; prev_button_down = button_down;

4 Anemometer design Your task is to design the measurement system for a hand-held anemometer. The measures wind speed with a traditional rotating cup system. Wind speed is determined by measuring the rotation period, scaling and displaying accordingly. You will likely find it helpful to read the rest of the exam before solving any of the parts. To make the measurement, you have at your disposal a simplified Smart Fusion kit hardware timer. device following and a Interface to the APB bus The kit has the following APB3 bus interface. The signal names are shown in bold. The ABP3 bus signals follow APB3 timing and protocol. Read and write cycles are provided on the next page. PSEL is configured to be 1 when memory locations 0x x are accessed. SmartFusion Level Sensitive Interrupt: INT APB Write Data: PWDATA (32 bits) APB Read Data: PRDATA (32 bits) APB Peripheral Write: PWRITE Peripheral Address: PADDR (8 bits) Peripheral Select: PSEL Bus Clock: PCLK Bus Ready: PREADY As shown, this system has a single interrupt that can be generated. It is level-sensitive and is named INT.

5 Hardware Timer The hardware timer will become the value on TDI if TWE is high on the positive edge of the timer clock. Otherwise the counter will increment every cycle. If the counter reaches its maximum value it wraps around to zero (i.e. it is a modulo counter). The current value of the timer is always available on TDO. Timer Timer Data In: TDI (32 bits) Timer Data Out: TDO (32 bits) Timer Write Enable: TWE Timer Clock: TC APB Timing diagram The following diagrams are provided as a reminder of the APB timing with no wait states. ABP3 Read Timing ABP3 Write Timing

6 Part 1: Hardware Timer APB3 Bus Interface (16 points) Provide a hardware interface between the SmartFusion APB3 bus and the hardware timer to allow the SmartFusion to read and write the counter. Assume the timer s counter register is at address 0x (and only that address). You may use standard gates such as ANDs, ORs, NOTs (as well as standard bubbles). Be sure to show all connections. You may use GND and VCC to indicate a logical 0 and 1 respectively. You may not use Boolean or Verilog expressions. You will lose points for having unneeded logic. Note: we have put all the APB connections on the left and all the Timer connections on the right. This means inputs and outputs are intermixed, be sure you are driving all outputs! [10 points]

7 Part 2: Anemometer Interrupt Hardware Interface (16 points) The anemometer provides a high pulse (signal name PULSE) that is several clock (PCLK) cycles wide and is not synchronized to the APB clock. We wish to use this signal to generate an interrupt and then use the counter to time the period of each anemometer rotation. The only interrupt line available on your processor is level sensitive and is active high. This means it will interrupt as long as it remains high. Design logic using standard gates and D flip-flops that will provide the level-sensitive interrupt when PULSE goes high. Recall you have to hold a level-sensitive interrupt true until it is cleared by the interrupt service return. o One D flip-flop is provided, though you may use additional ones as needed. Also, provide logic that will clear the interrupt when a write to memory address 0x (and only that address) occurs.

8 Part 3: Interrupt Service Routine (18 points) Write an interrupt service routine in C called anemometer_interupt that will calculate and display the wind speed by reading the time of a rotation, scaling and then displaying the result using your hardware designed above. Assume that you have a function called scale that will convert the time of rotation to KPH. Scale accepts time per rotation in microseconds. Assume that you have another function called display that accepts KPH and provides display. It is acceptable if the first speed value is incorrect. All subsequent values should be correct. Assume the APB bus runs at 1 MHz. Function Prototypes: (void) display (int kph) (int kph)scale (int ms) Providing comments may improve your chance for partial credit.