1 C and Embedded Systems A P-based system used in a device (i.e, a car engine) performing control...

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C and Embedded Systems

• A P-based system used in a device (i.e, a car engine) performing control and monitoring functions is referred to as an embedded system.– The embedded system is invisible to the user– The user only indirectly interacts with the embedded system

by using the device that contains the P

• Most programs for embedded systems are written in C– Portable – code can be retargeted to different processors – Clarity – C is easier to understand than assembly– compilers produce code that is close to manually-tweaked

assembly language in both code size and performance

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So Why Learn Assembly Language?

• The way that C is written can impact assembly language size and performance– i.e., if the uint32 data type is used where uint8 would suffice, both

performance and code size will suffer.

• Learning the assembly language, architecture of the target P provides performance and code size clues for compiled C– Does the uP have support for multiply/divide?

– Can it shift only one position each shift or multiple positions? (i.e, does it have a barrel shifter?)

– How much internal RAM does the P have?

– Does the P have floating point support?

• Sometimes have to write assembly code for performance reasons.

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C Compilation

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MPLAB PIC24 C Compiler

• Programs for hardware experiments are written in C

• Will use the MPLAB PIC24 C Compiler from Microchip

• Excellent compiler, based on GNU C, generates very good code

• Use the MPLAB example projects from reesemicro.com to get started

http://www.reesemicro.com/Home/pic24-software-library-collection

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Referring to Special Function Registers

PORTB = 0xF000;

#include "pic24.h"

Must have this include statement at top of a C file to include the all of the header files for the support libraries.

Special Function Registers can be accessed like variables:

extern volatile unsigned int PORTB __attribute__((__sfr__));

Special function register

Defined in compiler header files

In C code, can refer to special register using the register name

Register Name

__attribute__ is not ANSI C,but it is common in GCC

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Referring to Bits withinSpecial Function Registers

The compiler include file also has definitions for individual bits within special function registers. Can use these to access individual bits and bit fields:

PORTBbits.RB5 = 1; //set bit 5 of PORTB PORTBbits.RB2 = 0; //clear bit 2 of PORTB

if (PORTBbits.RB0) { //execute if-body if LSB of PORTB is '1'....}

OSCCONbits.NOSC = 2; //bit field in OSSCON register

Uses bit fields in C structs

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Referring to Bits withinSpecial Function Registers

Using registername.bitname requires you to remember both the register name and the bitname. For bitnames that are UNIQUE, can use just _bitname.

_RB5 = 1; //set bit 5 of PORTB _RB2 = 0; //clear bit 2 of PORTB

if (_RB0) { //execute if-body if LSb of PORTB is '1'....}

_NOSC = 2; //bit field in OSSCON register

Variable Qualifiers, Initialization

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uint16 u16_k; //initialized to 0uint8 u8_k = 4; //initialized to 4

_PERSISTENT uint8 u8_resetCount; //uninitialized, value // is unknown

If a global variable does not have an initial value, by default the runtime code initializes it to zero – this includes static arrays. To prevent a variable from being initialized to zero, use the _PERSISTENT macro in front of it:

The C runtime code is run before main() entry, so run on every power-up, every reset. Use _PERSISTENT variables to track values across processor resets.

Initialization to zero is required in C only for global variables

C Macros, Inline Functions

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The support library and code examples makes extensive use of C macros and Inline functions. The naming convention is all uppercase:

#define DEFAULT_BAUDRATE 57600

Macros, the left hand label is replaced by the right hand text

#define LED1 _RB15

static inline void CONFIG_RB1_AS_DIG_INPUT(){ DISABLE_RB1_PULLUP(); _TRISB1 = 1; _PCFG3 = 1;}

Inline functions expand without a subroutine call.

Macros are like an “inline” function

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PIC24HJ32GP202 µC

Hardware lab exercises will use the PIC24HJ32GP202 µC (28-pin DIP)

Note that most pins have multiple functions.

Pin functions are controlled via special registers in the PIC. Will download programs into the PIC24

µC via a serial bootloader that allows the PIC24 µC to program itself.

Copyright Delmar Cengage Learning 2008. All Rights Reserved.

From: Reese/Bruce/Jones, “Microcontrollers: From Assembly to C with the PIC24 Family”.

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Initial HookupThere are multiple VDD/VSS pins on your PIC24 µC; hook them all up!!!



Not included in your board.

Any input voltage from 5 V to 15 V will work.

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Powering the PIC24 µC

A Wall transformer provides 15 to 6V DC unregulated (unregulated means that voltage can vary significantly depending on current being drawn). The particular wall Xfmr in the parts kit provides 6V with a max current of 1000 mA.

The LM2937-3.3 voltage regulator provides a regulated +3.3V. Voltage will stay stable up to maximum current rating of device.

With writing on device visible, input pin (+9 v) is left side, middle is ground, right pin is +3.3V regulated output voltage.

The POWER LED provides a visual indication that power is on.

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Aside: How does an LED work?3.3V

470 ohm

Power onLED

Anode (long lead)

Cathode (short lead)

A diode will conduct current (turn on) when the anode is at approximately 0.7V higher than the cathode. A Light Emitting Diode (LED) emits visible light when conducting – the brightness is proportional to the current flow. The voltage drop across LEDs used in the lab is about 2V.

current limiting resistor

Current = Voltage/Resistance ~ (3.3v – LED voltage drop)/470 = (3.3v – 2.2V)/470 = 2.7 mA

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Reset

PIC24 µC

VDD

VSS0.1

MCLR#

10K ohm

Reset Switch

+

+3.3V

When reset button is pressed, the MCLR# pin is brought to ground. This causes the PIC program counter to be reset to 0, so next instruction fetched will be from location 0. All Cs have a reset line in order to force the C to a known state.

10K resistor used to limit current when reset button is pressed.

The Clock

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The PIC24 C has many options for generating a clock; can use an external crystal or internal oscillator.

We will use the internal clock.



Internal Fast RC Oscillator + PLL

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Our examples use this! Internal FRC + PLL configured for 80MHz.

RC Oscillators discharge a capacitor through a resistor

PLLPhased Locked Loop

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Configuration BitsConfiguration bits are stored at special locations in program memory to control various processor options. Configuration bits are only read at power up.

Processor options controlled by configuration bits relate to Oscillator options, Watchdog timer operation, RESET operation, Interrupts, Code protection, etc.

The file pic24_config.c file included by the sample programs used in lab specifies configuration bits used for all lab exercises.

We will discuss the meaning of the configuration bit options as it is necessary.

The PC Serial Interface

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We use a special USB-to-Serial cable to connect our board to the PC. This serial interface outputs 3.3 V levels compatible with the PIC24 µC pins (careful, most USB-to-Serial cables use +/- 9V levels!!).

The serial interface will be used for ASCII input/output to PIC24 µC, as well as for downloading new programs via the Bully Serial Bootloader (winbootldr.exe).

ledflash.c

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Testing your PIC24 SystemAfter you have verified that your hookup provides 3.3 V and turns on the power LED, the TA will program your PIC24 µC bootloader firmware. Use to program your PIC24 with the hex file produced by the echo.c program and verify that it works.

(a) Select correct COM port, baud rate of 230400, open the COM port.(b) Browse to hex file

(c) To program, press the ‘MCLR# and Prgm’ while power is on.

After downloading ‘echo.c’

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Welcome message printed by ‘echo.c’ on reset or power-on.

Status messages from bootloader

Type letters here and press ‘send’ to test, or type here.

If pin 6 on serial connector tied to MCLR#, then press this to download a program.

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Reading the PIC24 Datasheets• You MUST be able to read the PIC24 datasheets and find

information in them.– The notes and book refer to bits and pieces of what you need to

know, but DO NOT duplicate everything that is contained in the datasheet.

• The datasheet chapters are broken up into functionality (I/O Ports, Timer0, USART)– In each chapters are sections on different capabilities (I/O ports

have a section on each PORT).

• The PIC24 Family reference manual has difference sections for each major subsystem.

• The component datasheet for the PIC24HJ32GP202 has summary information, you will need to refer the family reference manual most often.

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PIC24 Reset



MCLR# -- external reset button brings input low causes reset.

RESET# instruction causes reset.

Power-on causes reset after voltage stabilizes.

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What RESET type occurred?

Bits in the RCON special function register tell us what type of reset occurred.



Watchdog Timer

V 0.1 25Copyright Delmar Cengage Learning 2008. All Rights Reserved.


WDT Uses

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Error Recovery: If the CPU starts a hardware operation to a peripheral, and waits for a response, can break the CPU from an infinite wait loop by reseting the CPU if a response does not come back in a particular time period.

Wake From Sleep Mode: If the CPU has been put in a low power mode (clock stopped), then can be used to wake the CPU after the WDT timeout period has elapsed.

Power Saving Modes

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Sleep: Main clock stopped to CPU and all peripherals. Can be awoke by the WDT. Use the pwrsav #0 instruction.

Idle: Main clock stopped to CPU but not the peripherals (UART can still receive data). Can be awoke by the WDT. Use the pwrsav #1 instruction.

Doze: Main clock to CPU is divided by Doze Prescaler (/2, /4, … up to /128). Peripheral clocks unaffected, so CPU runs slower, but peripherals run at full speed – do not have to change baud rate of the UART.

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General-purpose I/OThe simplest type of I/O via the PIC24 µC external pins are parallel I/O (PIO) ports.

A PIC24 µC can have multiple PIO ports named PORTA, PORTB, PORTC, PORTD, etc. Each is 16-bits, and the number of PIO pins depends on the particular PIC24 µC and package.The PIC24HJ32GP202/28 pin package has:

PORTA – bits RA4 through RA0 PORTB – bits RB15 through RB0These are generically referred to as PORTx.

Each pin on these ports can either be an input or output – the data direction is controlled by the corresponding bit in the TRISx registers (‘1’ = input, ‘0’ = output).

The LATx register holds the last value written to PORTx.

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PORTB Example

Set the upper 8 bits of PORTB to outputs, lower 8 bits to be inputs:

TRISB = 0x00FF;

Drive RB15, RB13 high; others low:

PORTB = 0xA000;

Wait until input RB0 is high:

while ((PORTB & 0x0001) == 0);

Wait until input RB3 is low:

while ((PORTB & 0x0008) == 1);

Test returns true while RB0=0 so loop exited when RB0=1

Test returns true while RB3=1 so loop exited when RB3=0

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PORTB Example (cont.)Individual PORT bits are named as _RB0, _RB1, .._RA0, etc. so this can be used in C code.

Wait until input RB2 is high:

while (_RB2 == 0);

Test returns true while RB2=0 so loop exited when RB2=1. Can also be written as:

while (!_RB2);

Wait until input RB3 is low:

while (_RB3 == 1) ;

Test returns true while RB3=1 so loop exited when RB3=0Can also be written as:while (_RB3);

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Switch Input

PIC24 µC

RB3

Vdd

10K External pullup

When switch is pressed RB3 reads as ‘0’, else reads as ‘1’.

If pullup not present, then input would float when switch is not pressed, and input value may read as ‘0’ or ‘1’ because of system noise.

PIC24 µC

RB3

don’t do this!

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PORTx Pin Diagram

TRIS bit controls tristate control on output driver

External pin shared withother on-chip modules

Reading LATx reads lastvalue written; readingPORTx reads the actualpin

LATx versus PORTx

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Writing LATx is the same as writing PORTx, both writes go to the latch.

Reading LATx reads the latch output (last value written), while reading PORTx reads the actual pin value.

PIC24 µC

RB3

Configure RB3 as an open-drain output, then write a ‘1’ to it.

The physical pin is tied to ground, so it can never go high.

Reading _RB3 returns a ‘0’, but reading _LATB3 returns a ‘1’ (the last value written).

LATx versus PORTx (cont)

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_LATB0 = 1;

_LATB1 = 1;

Compiler bset LATB,#0

bset LATB,#1

bitset/bitclr instructions are read/modify/write, in this case, read LATB, modify contents, write LATB. This works as expected.

_RB0 = 1;

_RB1 = 1;

Compiler bset PORTB,#0

bset PORTB,#1

bset/bclr instructions are read/modify/write – in this case, read PORTB, modify its contents, then write PORTB. Because of pin loading and fast internal clock speeds, the second bset may not work correctly! (see datasheet explanation). For this reason, our examples use LATx when writing to a pin.

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Aside: Tri-State Buffer (TSB)A tri-state buffer (TSB) has input, output, and output-enable (OE) pins. Output can either be ‘1’, ‘0’ or ‘Z’ (high impedance).

A

OE

Y YA

OE = 0, then switch closed OE = 1, then switch open

OE

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Bi-directional, Half-duplex Communication

PIC24 µC

Q

Q

DQ

D

DTRIS

rd_port

data busQ

Q

D Q

D

D TRIS

rd_port

data bus

PIC24 µC

0 1

PIC24 µC

Q

Q

DQ

D

DTRIS

rd_port

data busQ

Q

D Q

D

D TRIS

rd_port

data bus

PIC24 µC

1 0

driver enabled

driver disabled

driver disabled

driver enabled

Schmitt Trigger Input Buffer

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Each PIO input has a Schmitt trigger input buffer; this transforms slowly rising/falling input transitions into sharp rising/falling transitions internally.

PORTx Shared Pin Functions

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External pins are shared with other on-chip modules. Just setting _TRISx = 1 may be not be enough to configure a PORTx pin as an input, depending on what other modules share the pin: RB15 shared with AN9, which is

an analog input to the on-chip Analog-to-Digital Converter (ADC). Must disable analog functionality!

_PCFG9 = 1;

_TRISB15 = 1;

Disables analog function

Configure as input

_PCFG9 = 1;

_TRISB15 = 0;

Disables analog function

Configure as output

Analog/Digital Pin versus Digital-only Pin

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Pins with shared analog/digital functions have a maximum input voltage of Vdd + 0.3 V, so 3.6 V

Pins with no analog functions ( “digital-only” pins) are 5 V tolerant, their maximum input voltage is 5.6 V.

This is handy for receiving digital inputs from 5V parts.

Most PIO pins can only source or sink a maximum 4 mA. You may damage the output pin if you tie a load that tries to sink/source more than this current.

Internal Weak Pullups

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External pins with a CNy pin function have a weak internal pullup that can be enabled or disabled.

Change notification input; to enable pullup: CN11PUE = 1;To disable pullup: CN11PUE = 0;

Open Drain Outputs

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Each PIO pin can be configured as an open drain output, which means the pullup transistor is disabled.

_ODCxy = 1 enables open drain, _ODCxy = 0 disables open drain

_ODCB15 = 1; Enables open drain on RB15

Port Configuration Macros

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For convenience, we supply macros/inline functions that hide pin configuration details:

CONFIG_RB15_AS_DIG_OUTPUT();

CONFIG_RB15_AS_DIG_INPUT();

These macros are supplied for each port pin. Because these functions change depending on the particular PIC24 µC, the include/devices directory has a include file for each PIC24 µC, and the correct file is included by the include/pic24_ports.h file.

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LED/Switch IO: Count number of

press/releases

1 C and Embedded Systems A P-based system used in a device (i.e, a car engine) performing control...

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