Theory behind the control of Embedded System Peripherals

Programming the McVASH deviceM. Smith

University of Calgary

Many people like to sing in the shower. However, its rather boring as there is no

accompaniment . The McVASH device solves this program.

Microprocessor controlled Voice Activated Shower Head

Water propelled electrical turbine for safety Your voice is DSP analysed (Lab. 5)

and the rhythm detected. ◦ This controls the water level from the shower head.◦ The beat of the water on the side of the shower walls

provides the accompaniment to your singing!

McVASH (Advert)

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Every thing must be as small as possible to reduce cost. 1 cent / device saving over 1 million devices is a lot of extra profit!

Device control register – is a bit pattern◦ Bit to “turn on” McVASH device (read and write)◦ Bit to “turn on” Sound part (read and write)◦ Sound being recognized bit (microphone working bit)

(READ ONLY RO) NOTE: Written R Oh not R zero – in data booksEtc

◦ 16 bits (1 half-word, unsigned short int) can act as 16 different software switches to activate 16 different hardware operations.

Peripheral device registers needed to control device behaviour

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Every thing must be as small as possible to reduce cost. 1 cent / device saving over 1 million devices is a lot of profit!

Water temperature – value bit pattern◦ McVASH device has a TMP03 device (Part of ENCM511Lab. 2)

◦ Range -- Temperatures between 0C to 70 C◦ Accuracy – Report temperature to within 0.5C

◦ Typical value are 6 C, 15.5 C◦ 15.5 C is not an integer value◦ Floating point processors $400 each

◦ Need cheaper approach.◦ Use integer number to represent a limited number of floating point

values◦ Use ideas from ENCM369 on number representation

Peripheral device registers needed

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Water temperature – value bit pattern◦ Range -- Temperatures between 0C to 70 C◦ Accuracy – 0.5C is sufficient for a shower

◦ Store 0 C as value 0x00 b 0000 0000◦ Store 0.5C as value 0x01 b 0000 0001◦ Store 1 C as value 0x02 b 0000 0010◦ Store 6 C as value 12 (0x0C) b 0000 1100◦ Store 6.5 C as value 13 (0x0D) b 0000 1101

Store temperature X C as hex value 2X – now all temperatures stored as integer values on a $2 integer processor rather than a $400 floating point processor Note – integer processors CAN do floating point operations (e.g.

Blackfin); just not as quickly as FP processors

Peripheral device registers needed

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Numerical representation◦ Biggest value 70C = 140◦ Smallest value 0C = 0

Choices of how to store this value◦ Use unsigned byte (8 -bit) value:

min 0 to max 255◦ Use signed byte value :– min -128 to +127 –

NO◦ Use signed word value :- min -32000 to +32000

Useful in Canadian markets when out-door camping◦ Cheapest solution that works – unsigned byte

value

Water Temperature register

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Need timer to turn off the water◦ Need to count in seconds

We have a processor that has a clock that runs at 1 MHz◦ Would need divide logic to turn 1MHz clock into 1

second clock.

Cheaper to have 32 bit register that counts the 1 MHz clock◦ How many seconds can be counted this way?

Should we use a signed 32 bit register or an unsigned 32 bit register?

Which register type stores the largest count? (Quiz hint)

McVASH is a Green product!

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Save more money by having the “turn the timer on” control bit as one of the bits in existing “Processor control register”.

Need 32 bit counter register TCOUNT Counts how many “ticks” have occurred since time turned on

Need 32 bit “finish” register (TPERIOD). When the TCOUNT reaches TPERIOD, tell the system to do an interrupt

service routine. This ISR turns the water off.

Need 64 bit CYCLES register which counts how long the processor has been turned on◦ Cheaper to build as “2” 32 bit registers CYCLES and CYCLES2

◦ Design of McVASH timer very similar to Blackfin CORE TIMER

Timer registers needed

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Each McVASH device has◦ 3 32-bit timer registers◦ 8-bit temperature register (temp *2 is stored)◦ 16-bit control register

Sell on market – two products. Try to minimize total cost◦ Blackfin controlling 1 McVASH for small summer

cottages with only one shower◦ Blackfin controlling many McVASH for big houses

with many showers How do these ideas fit in with the

Microcontroller discussed in Monday’s lecture?

McVASH registers

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McVASH -- Summer Cottage version

CPU

containsCCUALU

data registersand

pointer registers

CONTROL BUS

ADDRESSBUS

DATA BUS

BOOTROM

Used at startup

Instruction(program)

ROM

McVASH

MemoryAddress0x20???

DataRAM

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McVASH -- Big House version

CPU

containsCCUALU

data registersand

pointer registers

CONTROL BUS

ADDRESSBUS

DATA BUS

BOOTROM

Used at startup

Instruction(program)

ROM

McVASH

MemoryAddress0x20???

PLUG and Play

McVASH

MemoryAddress0x30???

PLUG and Play

DataRAM

◦ Over a period of 5 seconds, keep the temperature of the water at 37C

◦ PSEUDO CODEFor ever

Record temperature values at regular intervals

For the last 5 seconds, work out temperature if below 36.5C add less cold water, more hot if above 37.5C add more cold water, less cold

end

Typical function for McVASH

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#define TEMPERATURE37C 74

// PROTOTYPES OF FUNCTIONS TO BE DEVELOPED void WaitTenthSecond( void);

unsigned ReadTemperatureASM( void);

unsigned int AverageTemperature(unsigned int *temperature); // temperature array passedOR unsigned int AverageTemperature(unsigned int temperature[ ] ); SAME CODE NEEDED

other prototypes as needed

void ControlWaterTemperature(void) { unsigned int temperature[50];

for (count = 0 to 50) { WaitTenthSecond( ); temperature[count] = ReadTemperatureASM( ); The code to be developed this lecture }

unsigned int averageTemperature = AverageTemp(temperature); THIS FUNCTION CAN’T POSSIBLY BE USED TO CALCULATE AVERAGE TEMPERATURE FROM AN ARRAY OF TEMPERATURES – WHY NOT?

if (averageTemperature != TEMPERATURE37C ) AdjustWaterTemperature(averageTemperature);

};

C/C++ code – First attempt

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temperature[count] = writes to a memory array – stored on the processor stack – SDRAM◦ The processor is started up with a stack (ENCM369)

already placed in memory◦ Easy to access this sort of array using “Standard C”

ReadTemperature – reads from a memory array of registers that belong to the McVASH device◦ Where is this peripheral device register array?

◦ Must know location of the peripheral device register array in order to be able to read and write values to it!

◦ There might be more than one memory array (one for each McVASH peripheral device)

Writing / Reading values

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MONDAY’s lecture:Every external device needs this amount of support “glue logic” to work

External Device

Device itself with all necessary internal logic

to do the things it needs to do

DATA BUS

OEOutput Enable other signals such as interrupt signals, etc

ADDRESS BUS

DECODE LOGIC• Address strobe• Data strobe• Read/Write

control

• CS – chip select

McVASH 1 – control logic built to recognize addresses on the processor address bus in the range 0x300000 to 0x30001F.◦ Why not the range 0x200000 to 0x2FFFFF?◦ Why not the range 0x500000 to 0x50001F?

McVASH 2 – control logic build to have the device register memory array start at address 0x400000

McVASH 3 – control logic build to have the device register memory array start at address 0x500000

System design – decides on control logic

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Each McVASH device decode logic permit◦ 3 32-bit timer registers

Address are 0x300000, 0x300004, 0x300008◦ 8-bit temperature register (temp *2 is stored)

Address is 0x30000C◦ 16-bit control register Address is ??????

Next location available is 0x30000D (address is not even – look at bit 0 -- 0xD = 1101)

but processors CAN NOT access 16-bit peripheral registers (or memory locations) whose address is “odd” (unless you want to pay more for the processor)

Control register therefore designed to have address 0x30000E (address is even – look at bit 0 )

McVASH #1 registers start at0x300000

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Each McVASH device has◦ 3 32-bit timer registers

Address are 0x400000, 0x400004, 0x400008◦ 8-bit temperature register (temp *2 is stored)

Address is 0x40000C◦ 16-bit control register Next location available is 0x40000D

(address is not even) but processors CAN NOT access 16-bit registers whose memory address is “odd” (unless you want to pay more for the processor)

control register therefore designed to have address 0x40000E

McVASH #2 registers start at0x400000

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Here is the code from Monday’s tutorial to add two values in an array

…. Other code

.section L1_data;.byte4 _fooArray[2];

.section program; .global _AddArrayValuesASM;_AddArrayValuesASM:#define sum_R0 R0 // register int sum;

sum_R0 = 0; // sum = 0;#define pointer_to_array_P1 P1 // register int * pointer_to_array

P1.L = lo(_fooArray); P1.H = hi(_fooArray); // pointer_to_array = &fooArray[0];R1 = [pointer_to_array_P1]; // int temp = fooArray[0]; sum_R0 = sum_R0 + R1; // sum = sum + temp R1 = [pointer_to_array_P1 + 4]; // temp = fooArray[1]; sum_R0 = sum_R0 + R1; // sum = sum + temp

_AddArrayValuesASM .END: RTS;

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Here is the first attempt at code for unsigned int ReadTemperatureASM( );

.section program; .global _ ReadTemperatureASM;_ ReadTemperatureASM; :#define temperature_R0 R0#define pointerMcVASHDevice_P1 P1#define ADDRESS_McVASH_DEVICE 0x30000

P1.L = lo(ADDRESS_McVASH_DEVICE); P1.H = hi(ADDRESS_McVASH_DEVICE);

// Need to read 8-bit temperature value – B for Byte – zero-extend to 32-bits

temperature_R0 = B[pointerMcVASHDevice_P1 + 0xC] (Z);

_ ReadTemperatureASM;.END: RTS;

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Here is the first attempt to code to read Device 2 temperature sensor

.section program; .global _ReadTemperatureDevice2ASM;_ReadTemperatureDevice2ASM:#define temperature_R0 R0#define pointerMcVASHDevice_P1 P1#define ADDRESS_McVASH_DEVICE2 0x40000

P1.L = lo(ADDRESS_McVASH_DEVICE2); P1.H = hi(ADDRESS_McVASH_DEVICE2);

// Need to read 8-bit temperature value – B for Byte – zero-extend to 32-bits

temperature_R0 = B[pointerMcVASHDevice_P1 + 0xC] (Z);

_ReadTemperatureDevice2ASM.END: RTS;22

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Add code to a test project to check syntax

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0x40000

unsigned int ReadTemperatureDeviceCPP(void) {

unsigned char temperatureValue; (byte)

Somehow read a byte value from a specific register from a device located some where on the external address bus of the microcontroller

return (unsigned int) temperatureValue;}

That “somehow read” operation is difficult to do with most “C/C++” compilers – which is why most interfaces to hardware are done in assembly code and nor written in “C” or “C++”

What would the code look if written in “C++”?

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#define ADDRESS_McVASH_DEVICE 0x30000

unsigned int ReadTemperatureDeviceCPP(void) {

unsigned char temperatureValue; // “byte” in “C” is unsigned char

// Use the key word “volatile” to tell embedded C” that this is an external device location

unsigned char *pointerToDeviceRegister = (volatile unsigned char *) ADDRESS_McVASH_DEVICE;

// Point to device temperature register pointerToDeviceRegister = pointerToDeviceRegister + 0xC;

// get the temperature value using a “pointer to a peripheral device register location”

temperatureValue = *pointerToDeviceRegister

return (unsigned int) temperatureValue;}

What would the code look written in Blackfin “C++”

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0x30000

0x30000

1. Use “print screen” to do a “VDSP screen capture” and then paste into a “.doc” file

2. After you have developed “C” code and linked it (without error)s, then “right click” on the source code window and select “MIXED”

◦ You can now see the assembly language code that the “C” compiler generated

3. You can cut and paste “example code” from the Powerpoint slides into VDSP is you know this trick

4. Starting point for my “Tests” on the McVASH device

Additional slides with hints for speeding up labs and assignments

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Compare our code and “C” codeproduced by the VDSP compiler

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P1 = HI_16 bits + lo_16bits= 0x2 * 0x10000 + 12 (0xC)

We have optimized the code (NO LINK or UNLINK, since we KNOW this is a LEAF function

Code has extra “strange characters” -- remove them

You are welcome to cut-and-paste code from my slidesBUT -- When cutting and pasting code from Powerpoint into VDSP

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