lab3

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16.3. Laboratory 3: The ARM parallel I/O ports

16.3.1. Objectives

The objectives of this laboratory are:

1. Configure the general purpose input-output of the ARM microcontroller to

match the circuitry available on the Keil ARM EVB.

2. To write a small program that monitors the input switches and controls the

LEDs on the EVB.

16.3.2. Introduction

Microcontrollers are used to monitor and control physical systems. Systems may be

very simple such as using a transducer to detect when the temperature reaches some

critical value and at that point drive some circuitry to turn on cooling. While the

system being monitored/controlled may be very complex the process can be simulated

by having the firmware monitor an on-off switch and depending upon the result

control an LED. In a real system the challenge for the hardware engineer is the

circuitry to interface the microcontroller to the external world while for the software

engineer the challenge is usually the magnitude of the number of inputs and outputs

required by a real system.

The ARM EVB has a set of LEDs wired to bits 8 to 15 of PortE plus 3 push button

switches labelled User (PB7), Tamper (PC13) and WakeUp (PA0) that may be used to

simulate user on-off inputs156

. See figure 16.10

Figure 16.10. ARM-switch interface

16.3.3. References.

156

To implement the Tamper and WakeUp functions the microcontroller alternative functions must be

implemented. Until this is done these two switches may be treated as general purpose input. The

joystick also provides a push function (pin PD11) so it could also be used.

Probe

Pin 10K

100nF

USER PB7

TAMPER PC13

RESET

1K 220K

WAKE UP

PA0

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1. STMicroelectronics: RM0008 Reference manual for the STM32F101xx,

STM32F102xx, STM32F103xx, STM32F105xx and STM32F107xx advanced

ARM-based 32-bit MCUs

Available on the Keil site as stm32f10_ref.pdf

2. STMicroelectronics: Data sheet for the STM32F105xx, STM32F107xx.

Connectivity line, ARM-based 32-bit MCU with 64/256 KB Flash, USB OTG,

Ethernet, 10 timers, 2 CANs, 2 ADCs, 14 communication interfaces

Available on the Keil site as stm32f105(7)_data.pdf

3. STMicroelectronics: STM32F10xxx Cortex-M3 programming manual.

4. Joseph Yiu. “The Definitive Guide to the ARM-Cortex M3” Second Edition.

Newnes

5. Chapter on ARM-IO in “Embedded Microprocessor Systems” by J.Kneen

16.3.4. Preliminary.

Section 16.3.9 gives the starting code for this laboratory. It includes turning the LEDs

on and off. The code is a smaller version of that used in the previous laboratories. In

the previous laboratories the LEDs flashed so quickly they could not be detected.

1. Part of this laboratory will require turning the LEDs on and off at a visible

rate. From your results to section 16.1.6 estimate the initial value of R0 in the

following code fragment to give a visible delay.

Delay ADDS R0,#-1 ;note minus 1

BNE Delay

To initialise R0 the code might be of the form “mov #constant”. To keep the

instruction within a 32 bit word157

the ARM limits the value of constant.

2. From reference 3 STM32F10xxx Cortex-M3 programming manual read

section 3.3.3 and document the restrictions on the value of constant.158

The USER switch in this laboratory is wired to GPIOB. This will require GPIOB to

be configured. (See preliminary 4)

3. Complete tables 16.11 and 16.12159

.

157

The ARM is a RISC reduced instruction set computer. It limits the number of instructions in order

to achieve higher speed. If 32 bit operands were allowed this would require two visits to memory

which would degrade the performance. 158

To overcome this restriction the instruction to initialise r0 will be ldr r0,=constant. 159

While students are only required to add code for GPIOB in this laboratory the additional

information is given for completeness and later reference.

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Port Base Address Clock Enable Bit in

RCC_APB2ENR

(address 0x40021018)

GPIOA

GPIOB

GPIOC

GPIOD

GPIOE

Figure 16.11. GPIO Base Address and Clock enables.

Register Mnemonic Address Offset

Configuration Register

Low

GPIOx_CRL

Configuration Register

High

GPIOx_CRH

Input Data Register GPIOx_IDR

Output Data Register GPIOx_ODR

Figure 16.12. Input output registers

Figure 16.13. Code to test switch.

4. Using the code provided in section 16.3.9 as a guide read through the chapter

on the ARM_IO and then develop the code to configure160

the pin/port

containing the user switch.

160

Note that the ARM internal clock is being used so the first step will be enabling the peripheral bus

clock to the port to be used.

Read the port with the switch.

Mask/isolate the switch bit.

Turn off LEDs –code given

Label Loop

Switch Pressed

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5. Develop the code to implement the flow chart of figure 16.13. Note in the

procedure this code will inserted after the LEDs have been turned on. The

LEDs will remain on when the switch is pressed.

16.3.5. Procedure

1. Start a new project in the Keil development environment. The Keil start up

code is not required. Save the project as Lab03.

Create the sample program Lab03.s given in section 16.3.9.

2. Insert the code from the preliminary for a visible delay and test. Use the MSO

to accurately measure your delay.

3. Modify your code to read the switch. Using the code as given the switch is

only read once. To test it will be necessary to either single step or hold the

switch down while re-running the code.

4. Modify your code to wait until the switch is pressed before turning the LEDs

off.

The existing program includes 4 fragments:

(i) Code to turn the LEDs on and off

(ii) Code to read an input switch

(iii) Code to make a decision on the state of the switch, and

(iv) A delay routine.

These fragments could be combined to form a simple state machine such as a

pedestrian road crossing. The actual traffic lights (red-green-yellow) would replace

the LEDs, the switch would be the request button and the delay would generate times

that match those required by a real pedestrian crossing.

5. Re-program your code to implement the design of figure 16.14.

6. Your report will be enhanced with a MSO trace of your program. Take a trace

with at least 3 channels:

(a) One channel probing the input switch

(b) A second probe on one of the first group of LEDs

(c) The third probe on one of the lower group of LEDs.

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Figure 16.14. Flashing LEDs

16.3.6. Questions

1. Often several outputs need to be wired together in an open drain/collector

configuration. What is open drain and how does it work? How may open drain

be configured on one of the ARM output pins?

2. In this laboratory the switch is sampled at one instant of time. If this happened

in, for example a pedestrian traffic light controller, the request could be

missed. What is needed is a circuit that remembers that a switch has been

pressed. It is suggested that the RS flip flop might be an appropriate

component. Explain how the RS flip flop could be implemented in the design

of a traffic light controller.

16.3.7. Report

As well as summarising what you did and answering the questions your report should

highlight any challenges the laboratory presented and how you overcame these.

Consider you are writing a laboratory guide for other students. Attempt to document

something to assist them with any problems you perceive they might have. Looking at

the footnotes might provide some ideas for the type of extra material you can include.

16.3.8. Looking ahead

For these laboratories assembly language has been used. In laboratory 16.4 using the

C language with the ARM microcontroller will be introduced. The laboratory will

also reinforce the work on the parallel ports by using a joystick to generate a range of

inputs which will allow programming of many output options.

16.3.9. Appendix

RCC_APB2ENR EQU 0x40021018

GPIOE_CRH EQU 0x40011804

GPIOE_ODR EQU 0x4001180c

Flash 4 LEDs in turn.

Flash other 4 LEDs.

Switch Pressed

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AREA RESET, DATA, READONLY

EXPORT __Vectors

__Vectors

DCD 0x20000200

DCD Reset_Handler

AREA |.text|, CODE, READONLY

EXPORT Reset_Handler

;

Reset_Handler PROC

mov r6,#0x40 ;enable clock to port E

ldr r7,=RCC_APB2ENR

str r6,[r7]

mov r6,#0x33333333 ;configure port E - o/p to LEDs

ldr r7,=GPIOE_CRH

str r6,[r7]

;

mov r6,#0xff00; ;turn LEDs on

ldr r7,=GPIOE_ODR

str r6,[r7]

;Insert your code here

mov r6,#0x00; ;turn LEDs off

ldr r7,=GPIOE_ODR

str r6,[r7]

Loop

b Loop

ENDP

END

lab3

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