ECS642U Embedded Systems Digital I/O William Marsh.

ECS642U Embedded Systems

Digital I/O

William Marsh

2ARM University ProgramCopyright © ARM Ltd 2013

Acknowledgement

•Some slides from ARM University Program lab-in-a-box•Copyright acknowledged

Outline

• Principles of memory mapped I/O• Using Keil μVision

– Compile, Assemble, Link, Load– Memory map– Debug

• KL25Z memory layout• GPIO Configuration and use

– Bus Matrix– Pin multiplexing– Data direction– GPIO ports

Using KeilμVision

Compiling a Program

• Compile C program assembly code

• Link with libraries memory image

• Download

• Debug– Set breakpoints– Monitor variables

PCFreedom KL25Z

SDA debug

Compile and Assemble

• User program C code• Start initialisation in assembler code

– Symbolic machine code

• Useful to be able to read

Download

• MCU program held in Flash Memory– Non-volatile

• Flash memory also contains ‘loader’ program

• Once downloaded, program stays• Runs on reset button

Debugging

• ‘Debug connector’ on Freedom Board

• Breakpoint– Stop program at a

program address

• Monitor variable– Observe global

variable value

• …

Memory Mapped I/O

Memory Mapped I/O

• I/O Ports ‘alias’ memory locations• Load and store operations used for I/O• C program:

– Variable– ‘Volatile’

Cortex M0+ MCU Memory Map

0x0000_0000

0x0001_FFFF

16 KB SRAM

128KB Flash

0x2000_0000

0x2000_2FFF

0x1FFF_F000

SRAM_U (3/4)

SRAM_L (1/4)

KL25Z128VLK4

Memory Areas

• Flash memory– 0x0000 0000 – 0x0001 FFFF– 128 KByte– Used for program code

• SRAM– Data (or code)– 16 KBytes: 0x1FFF F000 - 0x2000 2FFF

General Purpose I/O

Overview

• How do we make a program light up LEDs• GPIO – General Purpose I/O

– Basic Concepts– Port Circuitry– Control Registers– Accessing Hardware Registers in C– Clocking and Muxing

• Circuit Interfacing– Inputs– Outputs

• Additional Port Configuration

Basic Concepts

• GPIO– Input: is input signal is a 1 or a 0?– Output: set output to 1 or 0

• Can use this to interface with external devices– Input: switch– Output: LEDs

3 Colour LED on the KL25Z

board is active low

Output Example: Driving LEDs

• Safe current for– LED and– MCU port driver

• Use resistor– R = (VDD – VLED)/ILED

• Set ILED = 4 mA• VLED depends on LED colour

– Red: ~1.8V Blue: ~2.7 V

• Solve for R given VDD = ~3.0 V– Red: 300Ω Blue: 75Ω

KL25Z GPIO Ports

• Port A (PTA) to Port E (PTE)– 32 bits

• Not all port bits are available– Depends on

package pin count

GPIO Port Bit Circuitry in MCU

• Configuration– I or O– Pin control

• Writing– (Value)– Clear– Set– Toggle

• Reading– Value

PDOR select

PDIR select

PDDR select

Data Bus bit n

Port Data Direction Register

D Q

Port Data Output

RegisterD Q

AddressDecoder

Address Bus

Pin or Pad on

package

Port Data Input

RegisterD Q

I/O Clock

Tgl

Rst

SetPSOR select

PCOR select

PTOR select

Pin Control Register

MUX field

Control Registers

• One set for each port– 32 bits

PDOR select

PDIR select

PDDR select

Data Bus bit n


D Q

Port Data Output

RegisterD Q

AddressDecoder

Address Bus

Pin or Pad on

package

Port Data Input

RegisterD Q

I/O Clock

Tgl

Rst

SetPSOR select

PCOR select

PTOR select


MUX field

PDDR: Port Data Direction

• Each bit can be input or output

• Input: 0• Output: 1• Reset clears

port bit direction to 0

PDOR select

PDIR select

PDDR select

Data Bus bit n


D Q

Port Data Output

RegisterD Q

AddressDecoder

Address Bus

Pin or Pad on

package

Port Data Input

RegisterD Q

I/O Clock

Tgl

Rst

SetPSOR select

PCOR select

PTOR select


MUX field

Writing Output Port Data

• Direct– value to

PDOR

• Toggle– 1 to PTOR

• Clear (to 0): – Write 1 to

PCOR

• Set (to 1): – Write 1 to

PSOR

PDOR select

PDIR select

PDDR select

Data Bus bit n


D Q

Port Data Output

RegisterD Q

AddressDecoder

Address Bus

Pin or Pad on

package

Port Data Input

RegisterD Q

I/O Clock

Tgl

Rst

SetPSOR select

PCOR select

PTOR select


MUX field

Reading Input Port Data

• Read from PDIR

Connecting a GPIO Signal to a Pin

• Multiplexer alternative use of pins • What should pin be connected with

internally?• Each configurable pin has a Pin Control

Register

PDOR select

PDIR select

PDDR select

Data Bus bit n


D Q

Port Data Output

RegisterD Q

AddressDecoder

Address Bus

Pin or Pad on

package

Port Data Input

RegisterD Q

I/O Clock

Tgl

Rst

SetPSOR select

PCOR select

PTOR select

Freescale: is the pin mux location in this diagram accurate?


MUX field

Alternative Pin Configuration

• Each pin has multiple uses

Pin Control Register (PCR)• Each pin has a PCR

• Also controls interrupts

Pin Control Register (PCR)

• 3 bit control multiplexing – what is pin used for?

Enable the Clock

• Enable clock to GPIO module– GPIO modules Disabled to save power– Using an unclocked module fault

• Control register SIM_SCGC5 – Clocks to GPIO ports– Enable clock to Port A

• Header file MKL25Z4.h has definitions

Bit Port13 PORTE12 PORTD11 PORTC10 PORTB9 PORTA

SIM->SCGC5 |= (1UL << 9);

SIM->SCGC5 |= SIM_SCGC5_PORTA_MASK;

Summary of Configuration

• Enable clock to GPIO port• Set pin to GPIO function

– MUX = 1

• Set direction of pin(s) on GPIO port– Some pins input– Some pins output

Review of Configuration Code

Config Code – Review

• Enable clock

• Note– bitwise or of two masks– |= used to leave other bits unchanged

// Enable clock to ports B and D

SIM->SCGC5 |=

SIM_SCGC5_PORTB_MASK | SIM_SCGC5_PORTD_MASK;

Config Code – Review

• Pin multiplexing: selecting GPIO

• Why 2 assignments?

// Make 3 pins GPIO

PORTB->PCR[RED_LED_POS] &= ~PORT_PCR_MUX_MASK;

PORTB->PCR[RED_LED_POS] |= PORT_PCR_MUX(1);

PORTB->PCR[GREEN_LED_POS] &= ~PORT_PCR_MUX_MASK;

PORTB->PCR[GREEN_LED_POS] |= PORT_PCR_MUX(1);

PORTD->PCR[BLUE_LED_POS] &= ~PORT_PCR_MUX_MASK;

PORTD->PCR[BLUE_LED_POS] |= PORT_PCR_MUX(1);

Config Code – Review • Make ports output:

– data direction register

// Set ports to outputs

PTB->PDDR |= MASK(RED_LED_POS) | MASK(GREEN_LED_POS);

PTD->PDDR |= MASK(BLUE_LED_POS);

Summary

• Memory mapped I/O has registers for:– For configuring operation– For doing I/O

• Most pins are multiplexed– All pins are GPIO

• Also have to configure clocks signal to GPIO Port

ECS642U Embedded Systems Digital I/O William Marsh.

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