MCU Interfaces

December 2, 3 2014

Politecnico di Milano, “Advanced Operating Systems”

David Siorpaes

Outline

• STMicroelectronics presentation

• Microcontroller Input/Output

• Analog to Digital Converter interface

• Buses overview and practical examples

• SPI for interfacing with an accelerometer: code review and demo

• Alternate Functions

• I2C for driving a port expander and interfacing with a NFC memory

• UART for driving a Bluetooth module: code review and demo

• Bluetooth module driver hands-on

• Development and debugging tools

2

All code and documentation is available

from Google Code Project:

https://code.google.com/p/stm32-boilerplate/

3

• A global semiconductor leader

• The largest European semiconductor company

• 2013 revenues of $8.08B

• Approx. 45,000 employees worldwide

• Approx. 9,000 people working in R&D

• 12 manufacturing sites

• Listed on New York Stock Exchange, Euronext Paris

and Borsa Italiana, Milano

Who we are 4

As of December 31, 2013

Where you find us 5

Our automotive products

are making driving safer,

greener and more

entertaining

Our smart power products

are allowing our mobile products to operate longer

and making more of our energy resources

Our MEMS & Sensors

are augmenting

the consumer experience

Our Microcontrollers

are everywhere

making everything smarter

and more secure

Our digital consumer products

are powering the augmented

digital lifestyle

Example consumer electronics products

• MEMS devices (accelerometer, gyroscopes, magnetometer, pressure

sensor) in Samsung Galaxy line smartphones

• STM32 MCUs in Microsoft Xbox controller

• MEMS devices in Nintendo controllers

• Telepass device for Autostrade s.p.a.

• Fitbit fitness and health trackers

6

Digital

Convergence

Group

(DCG)

Imaging,

BiCMOS, ASIC

& Silicon

Photonics

(IBP)

Automotive

Product Group

(APG)

Analog, MEMS

& Sensors

(AMS)

Microcontroller,

Memory &

Secure MCU

(MMS)

Embedded Processing Solutions

(EPS)

Industrial &

Power Discrete

Group

(IPD)

Product Segments 7

Sense & Power and Automotive

Products (SP&A)

Effective Q4 2014, ST discontinued its commodity camera module

products and DCG and IBP product groups merged to form DPG.

ST Central Lab Organization

• We belong to a cross-division organization: Advanced Systems

Technology with specific focus on embedded software

• We provide to our internal customers early engineering, prototyping

and integration solutions of emerging software technologies on STM

products.

• Main competence areas

• Firmware design and development on STM32 microcontrollers

• Platform integration, benchmark and optimization

• Linux and Android OSes

• Current activities

• Optical Heart Rate Monitoring application

• Open Development Environment initiative for STM32 MCUs portfolio

8

MCU interfaces

A complex system… 10

Gyroscope

Accelerometer

Magnetometer

Pressure Sensor

Light Sensor

Camera

Touchscreen

GPS

NFC

Battery Gauge

MCU Applications

• Typically, MCU applications implement this simple common

architecture:

• Sense

• Compute

• Communicate and/or actuate

11

Sense Compute Communicate/

Actuate

A simple activity tracker example 12

3.3V

SPI/I2C S

PI, U

AR

T, I2

C, G

PIO

ADC

Accessing MCU interfaces

• MCU peripherals are exposed to programmer through a set of

memory mapped registers

• Registers are used to

• Configure peripheral (e.g.: speed, data format, interrupt generation,…)

• Data I/O: by reading/writing selected registers data is received/sent from/to the

peripheral

• Status indication: Transmission Completed, RX FIFO not empty,…

• Access to registers is abstracted by C libraries

13

MCU data transfer schemes

• Poll

• MCU actively polls peripheral status bits waiting for data to be sent/received

• Interrupt

• MCU is interrupted as data is received or data transmission is completed

• DMA

• MCU delegates data transfer to a dedicate Direct Memory Access engine

14

Analog sensors

• Simplest sensors are analog, i.e.: information is expressed as a

voltage or current

• Temperature sensors

• Gas sensors

• Photodiodes, phototransistors, photoresistors,...

• Analog to Digital Converter (ADC) is used to acquire data

15

MCU Analog Sensor

Buffer,

Filter,

Amplifier,

TIA,...

Voltage A

D

C

4 bit ADC 16

Time

Vref-

Vref+

V

0

1

2

3

4

5

6

7

8

9

10

11

12

13

15

14

ADC

• STM32F4 features 3 ADC with 16 channels each

• 12 bit resolution

• Adjustable Vrefs

17

Digital Sensors

• Most sensors have digital nature

• ADC conversions are performed onboard

• Data is conveyed digitally by means of a BUS

18

MCU Digital Sensor I

F

BUS

What is a bus ?

• A bus is a bunch of wires used to transfer digital information from one

device to another

• A bus definition includes a set of electrical, timing and protocol rules

that describe how to convey information from A to B

• Trivial examples:

• GPIO that conveys one single bit of information: on and off

• Eight GPIOs + strobe bit convey a byte of information. Used by old printer parallel

ports

19

Buses taxonomy

Buses categories

• Serial vs Parallel

• Synchronous vs Asynchronous

• Master/Slave vs uniform roles

• On-chip vs off-chip

20

Buses Examples

• USB

• SCSI

• UART, I2C, SPI

• AMBA

• CSI, DSI

• Display Port, HDMI

• CAN

Buses for embedded applications

• We will focus on the most important buses typically used in

embedded applications to connect the microcontroller to external

peripherals

• Sensors: e.g.: accelerometers, gyros, magnetometers, pressure sensors,…

• Actuators/UI: (e.g.: LED drivers, Port Expanders, displays)

• Storage (e.g.: SD-Cards, serial memories,…)

• Basic and widely used buses are:

• UART

• SPI

• I2C

21

SPI bus

SPI BUS

• Serial Peripheral Interface. Developed by Motorola

• Master/Slave bus. Communication is always initiated by ‘Master’

device and is bi-directional

• Full-duplex synchronous bus: a clock signal triggers data transfer

• Four wire bus:

• SCLK: Clock

• MISO: Master Input Slave Output

• MOSI: Master Output Slave Input

• CS (or SS): Chip Select (or Slave Select). Out of Band addressing

23

STM32 LIS3DH

SPI BUS implementation

• Very simple hardware implementation: master and slave shift

registers are clocked synchronously to drive MISO/MOSI lines

• At every clock pulse, a bit is exchanged between Master and Slave

even if no information exchange is needed by one of the two sides

• After eight clock periods, Master and Slave registers are swapped

24

How SPI works

• When Master wants to address the Slave it asserts the Chip Select

signal and starts driving the clock signal

• For both directions:

• Data is sampled on Clock Rising edge

• Data is propagated on Clock Falling edge

25

CS

SCLK

MISO

MOSI

Interfacing with the device

• SPI just provides raw, bi-directional data communication mean

• Communication protocol on top of SPI is defined by the specific

device

• However, a very common protocol is to use a register-based data

communication scheme:

• Device specifies a set of “registers” for implementing control and data paths

• Master reads/writes such registers using well defined SPI communication patterns

• Let’s have a look at the LIS3DH accelerometer specification…

26

LIS3DH accelerometer

• An accelerometer senses mechanical accelerations on three axis

• Applications: Motion activated functions, Free-fall detection,

Click/double click recognition, Pedometer, Display orientation,

Vibration monitoring and compensation,…

27

LIS3DH register map 28

Control

Data

Writing data to sensor 29

CS

SCLK

MOSI

Mode + Addressing Data

Reading data from sensor 30

CS

SCLK

MOSI

MISO

SPI in action: LIS3DH

accelerometer driver (poll mode)

Low level SPI Access 32

STM32

Standard Peripheral Library/Cube

LIS3DH driver

Low Level SPI access

Application

spiWrite() 33

spiRead() 34

LIS3DH driver 35

STM32

Standard Peripheral Library/Cube

LIS3DH driver

Low Level SPI access

Application

LIS3DH Driver

• Code review

• LIS3DH driver APIs

• Application

• Demonstration with STM Studio

36

Alternate Functions

Pin Routing 38

PB11: CS

PB13: SCK

PB14: MISO

PB15: MOSI

Slave Master

39

• Modern MCUs are

very complex and rich of

peripherals

• But MCU pins are a very

scarce resource…

MCU IO Ports 40

Alternate Functions

• Most MCU pins can have different mappings

• GPIO Input, GPIO Output

• Analog Input, Analog Output

• Alternate Function: connection to internal peripherals

• See page 44 of the STM32F4 specification

41

I2C bus

The I2C bus

• SPI is a very common way to interface with devices but has one

severe limitation: N slaves implies 3 + N wires

• More MCU pins

• More complex PCB design

43

[…]

The I2C bus

• Inter Integrated Circuit (I2C) is a standard developed by Philips in

1982 that defines Inter-Chip communication protocol

• Two-wire low speed communication protocol. Data and Clock signals

• Synchronous communication.

• Master-Slave architecture, with possibility to address multiple slaves

“in-band” (vs Out of Band like SPI). 7-bit addresses

44

Pull UP resistors

I2C principles • Communication is always initiated

by the Master. Master drives clock

• Start/Stop conditions define

communication boundaries:

• Start: SCL HI, SDA falling

• Stop: SCL HI, SDA rising

• These are the only two cases when

SDA is allowed to toggle with SCL HI

• Data is transferred by driving SDA

line during SCL LOW.

• SDA is sampled on SCL rising

edge

45

I2C principles

• Master initiates an I2C transaction with a START condition

• The 7-bit address of the device follows the START condition

• LSB indicates whether the Master intends to perform a Read or Write

operation from/to the device

• The addressed device replies with an ACK/NAK bit

• The communication from now on is device specific and usually

includes addressing and reading/writing internal registers (like SPI)

• Data recipient (Either Master or Slave) acknowledges 8 bit

transactions via ACK bit

• Let’s see some examples...

46

NXP’s PCF8574 port expander example

• GPIO pins are a very limited resource in MCU designs

• Port Expanders devices are used to:

• Extend GPIOs availability

• Enable higher current driving capabilities (e.g.: LED drivers for backlight)

• Pulse Width Modulation (PWM)

• “Remotize” actuation block from controller block (e.g.: display, touch sensor)

• Drives up to eight GPIOs

• Configurable address (three free bits)

• No internal registers

47

PCF8574 write example 48

Address

Fixed

part

Address

configurable

part

The hardware/software interface 49

Send Start Sequence

Send Slave Address

Wait for Slave ACK

Send data

Wait for Slave ACK

Send Stop Sequence

Wait for Start OK

M24LR NFC memory read example 50

M M M S S M M

M M M S M M M S S S M M S M M

Repeated Start

UART

UART bus

• Derives from old fashioned TTY terminals and modems

• Very simple bus: just two wires

• Asynchronous interface: both ends must agree on baud rate

• A logical ‘1’ is represented as a high signal level

• Start and stop conditions mark 8-bit data boundaries

52

Start condition Stop condition Data Frame

0

1

Time

A B TX

TX RX

RX

The hardware/software interface

• A ‘write’ operation to Data Register triggers data emission on TX pin

• A ‘read’ operation from Data Register returns the last byte on wire

• Status registers (aka flags) convey information on transmission status

• TXE: Transmit data register empty

• RXNE: Read data register not empty

• FR: Framing Error

• Interrupts can be associated to above flags

• DMA can be used on both TX and RX side

• Let’s see the Bluetooth driver as an example

53

UART in action, IRQ and DMA mode:

hooking up a Bluetooth module to

STM32

Bluetooth introduction

• Bluetooth is a low range low data rate wireless transmission protocol

• Operates in the 2.4GHz ISM band

• Bluetooth SIG standardization body defines:

• Radio physical link layer

• MAC

• Service discovery

• Application profiles

55

Bluetooth protocol stack 56

Radio

Baseband

Link Manager

Audio

Control

HCI interface

L2CAP

SDP TCS RFCOMM

SPP BNEP OBEX

Required

Optional

Audio

App

Management

Apps

Phone

Apps

IP

Apps vCard

No standard

Network

stack (HW)

Network

stack

(SW or HW)

Application

Layer (SW) Generic

Apps

Stack runs on host. Typical in PC world 57

Bluetooth Radio

Protocol Stack

Application

HCI interface

UART/SPI/USB/…

C Software interface

e.g.: socket

Stack runs on external module 58

Bluetooth Radio

Protocol Stack

Application

Adapter IF

HCI interface

SPI

AT Commands

UART

Bluetooth

Module

Comparing the two approaches

Stack on Host

• No extra MCU, low cost

• Finer grained control, easy to

upgrade, debug, patch

• Bigger footprint

• Flash and RAM

• MCU cycles

• More complex integration

• Usually closed source

• Toolchain/OS considerations

• Real time constraints

Stack off host

• Can be driven by very simple and

low cost MCUs

• Very easy to integrate. No

dependencies with OS and

Toolchains

• Usually supports limited profiles

(Typically SPP only)

• Software needs to manage

control/data path split

59

SPP Profile

• SPP stands for Serial Port Profile. It is meant to act as a bare Serial

Port found in Windows (e.g.: COM1) or Linux (e.g.: /dev/ttyS0)

• Very simple mean to make an embedded device to communicate to a

host system (PC or phone)

• Does not define any protocol or data format on top of plain serial. This

is left to application/middleware developer (cfr.: application profiles

defined in BTLE)

60

SPBT2632 Bluetooth module

SPBT2632 main features

• On module Bluetooth stack

• Data path and control path on UART bus

• Enables SPP profile and, optionally, audio SCO channels

• Seven GPIOs

62

SPBT2632 main features 63

SPBT2632 modes

• Data path and control paths are not

split using registers as seen before

• SPBT2632 and most Bluetooth SPP

modules on market feature two

different “modes”:

• Command mode: module is

disconnected. Accepts commands

and sends events

• Data mode: module is connected.

All data on the interface port is

transmitted/received on/from air

• But… we just have one UART port !

64

See page 10

of the spec

SPBT2632 state machine 65

stm SPBT2632C2A-StateMachine

Reset

Command/DisconnectedBypass/Connected

Command/Connected

[Device Disconnected]

/AT-AB

ConnectionDown

[AT+AB Bypass]

/AT-AB

-BypassMode-

[^#^$^% plus two seconds idle]

/AT-AB -CommandMode-

[Device Disconnected]

/AT-AB

ConnectionDown

[Device Paged]

/AT-AB ConnectionUp 38e7d8e26fdd

AT-AB -BypassMode-

Simplified State Machine 66

Start

Disconnected Connected

Unknown

Reset/”AT-AB BDAddress XXXXXXXX”

Connection/”AT-AB -BypassMode-”

Disconnection/”AT-AB ConnectionDown”

Simple SPP Bluetooth driver review

Simple Bluetooth driver code review

We will cover:

• Initialization of needed peripherals

• UART port signals

• GPIO for module reset

• Usage of basic Operating System synchronization mechanisms

(Using RTX OS from ARM)

• DMA mode UART transmission

• IRQ mode UART reception

68

Before coding…

Use case

• Wireless debugging and logging

facility for a wearable fitness device

• Connection is always initiated by

client. I.e.: module is Bluetooth

slave

• Data is collected by a

Linux/Windows PC or Android

smartphone for real-time

visualization and logging

Requirements

• Data flow in downstream only. Must

support up to 20 32-bit channels at

100Hz

• Shall consume as little processing

power as possible

• No stringent fault tolerance

69

Let’s see what the sample application does.. 70

Driver APIs 71

Interfacing with the host

Client side APIs

• Windows

• Configuration differs among Windows OS version and vendor drivers

• Provides high level access to SPP profile via COM port emulation

• APIs are identical to serial port communication

• Linux

• Uses ‘HCI tools’ for device configuration, pairing, etc. Unified interface

• Uses UNIX socket abstraction to establish connections and transfer data OR

• Uses ‘rfcomm’ utility to expose a UART like device like Windows

• Android

• Android Java APIs are basically a plain wrapper of Linux C APIs

73

Development and debugging

tools

Alternate Function mapping 75

CubeMX

• CubeMX helps configuring MCU pins alternate functions and

generates relevant initialization code for you

76

CubeMX

• CubeMX generates Clock Tree initialization code according to your

application requirements

77

Logic Analyzer

• A logic Analyzer is a debugging

instrument to analyze and decode

digital signals

• Very useful to debug low level

drivers

• Misconfigured pins and/or ports

• Protocol mismatches

• Timing conditions

78

ST-Link Utility

• Low profile debugger, freely

available from ST site

• Features:

• Program/Verify/Download Flash

memory

• Reset MCU, step by step execution

• ITM port. Very useful for immediate

printf-like facility

79

STM32 Studio

• Java application that allows to

monitor and display in real time

application variables

• Freely available from ST site

80