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EE 319K�Introduction to Embedded Systems

Lecture 9b: Decimal to ASCII conversion, Recursion, Busy-wait, and LCD interfacing

Bard, Cuevas, Erez, Gerstlauer, Holt, Valvano, Yerraballi, Telang, Tiwari

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Agenda

Review

Local variables with binding
Stack frames

Agenda

Decimal to ASCII conversion
Recursion (EE319H)
Busy-wait
LCD Interfacing

Bard, Cuevas, Erez, Gerstlauer, Holt, Valvano, Yerraballi, Telang, Tiwari

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Output

Output an integer.

Assume integer, n, is between 0 and 9999.

1) LCD_OutChar(0x30+n/1000) ;thousand’s digit

2) n = n%1000�3) LCD_OutChar(0x30+n/100) ;hundred’s digit

4) n = n%100�5) LCD_OutChar(0x30+n/10) ;ten’s digit

6) LCD_OutChar(0x30+n%10) ;one’s digit

Bard, Cuevas, Erez, Gerstlauer, Holt, Valvano, Yerraballi, Telang, Tiwari

This is not the solution to Lab 7 because 42 is output as “0042”

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Output Fixed point

Output a fixed-point decimal number.

Assume the integer part of the fixed-point number, n, is between 0 and 999, and resolution is 0.01.

1) LCD_OutChar(0x30+n/100) //one’s digit

2) n = n%100�3) LCD_OutChar(0x2E) //decimal point

4) LCD_OutChar(0x30+n/10) //tenth’s digit

5) n = n%10�6) LCD_OutChar(0x30+n) //hundredth’s digit

Bard, Cuevas, Erez, Gerstlauer, Holt, Valvano, Yerraballi, Telang, Tiwari

This is not the solution to Lab 7 because 42 is output as “0.042”

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Decimal to ASCII

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Decimal to ASCII

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Decimal to ASCII

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Recursion using the stack (EE319H)

; Input R0 is n ; Output R0 is return value n EQU 0 ;input parameter Fact PUSH {R0,LR} CMP R0,#1 BLS base SUB R0,#1 ;n-1 BL Fact ;Fact(n-1) LDR R1,[SP,#n] MUL R0,R0,R1 ;n*Fact(n-1) B done base MOV R0,#1 done ADD SP,#4 ;deallocate POP {PC}	uint32_t Fact(uint32_t n) { if(n<=1) return 1; return n*fact(n-1); }

Recursion requires putting parameters and locals on the stack

Bard, Cuevas, Erez, Gerstlauer, Valvano, Yerraballi, Telang, Tiwari

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Recursive Solution (EE319H)

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Recursive Solution (EE319H)

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Recursive Solution (EE319H)

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Input/Output Synchronization

Processor-Peripheral Timing Mismatch

Peripherals, e.g., displays, sensors, switches, generally operate MUCH slower than processor instruction times

Processor ~ MHz
Peripheral ~ kHz or Hz

MANY instructions can be executed while peripheral processes information

Bard, Cuevas, Erez, Gerstlauer, Holt, Valvano, Yerraballi, Telang, Tiwari

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Input/Output Sync. (cont.)

INPUT

OUTPUT

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What to do while the peripheral is BUSY?

BLIND CYCLE TRANSFER

Suppose that a BUSY control signal is not available
Perform I/O operation
Wait for a period of time that is guaranteed to be sufficient for operation to complete
Initiate next operation

I/O Sync Options (1)

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What to do while the peripheral is BUSY?

BUSY-WAIT (e.g., ready-busy, test-transfer)

Poll peripheral status – wait for READY/NOT BUSY
Perform other tasks between polls
Unless timed correctly, under/over run possible

One solution: POLL CONTINUOUSLY

I/O Sync Options (2)

Bard, Cuevas, Erez, Gerstlauer, Holt, Valvano, Yerraballi, Telang, Tiwari

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What to do while the peripheral is BUSY?

INTERRUPT/TRANSFER

Hardware INTERRUPTS processor on condition of READY/NOT BUSY
Facilitates performing other – background - processing between I/O transfers

Processor changes context when current transfer complete
Requires program structure to process context change

I/O Sync Options (3)

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Latency (EE319H)

Software latency or interface latency

Time from when new input is ready until time software reads data.
Time from when output is idle until time software writes new data.
Execute tasks at periodic intervals, latency is delay from when it should run until it does run

Interrupts guarantee an upper bound on the software response time

Count maximum time running with I=1, plus
Time to process the interrupt.

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Real-Time System (EE319H)

Real-time system

a system that can guarantee a worst case latency

Throughput/bandwidth

maximum data flow (bytes/s) that can be processed by the system

Priority

determines the order of service among two or more requests

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I/O Sync Options (EE319H)

What to do while the peripheral is BUSY?

DIRECT MEMORY ACCESS TRANSFER

Special purpose hardware logic monitors status of BUSY signal and maintains addresses of data to be communicated

Requires address and block size initialization

On the condition of NOT BUSY logic communicates next data element and increments address
When transfer is complete, logic provides COMPLETE INTERRUPT

Our TM4C123 supports DMA (but EE319K doesn’t use it)

Bard, Cuevas, Erez, Gerstlauer, Holt, Valvano, Yerraballi, Telang, Tiwari

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Sitronix ST7735 LCD

Resolution: widthxheight of 128x160 pixels
1.8” TFT LCD display with 18-bits per pixel
On-chip Display Data RAM 128x160x18bits
Device driver library ST7735.c provided to you – Implements the SPI protocol
Interfaced using the SPI protocol with 8 wires
Built-in micro-SD card for storage with 2 more wires

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// pin 10 Backlight +3.3 V

// pin 9 MISO unconnected

// pin 8 SCK PA2 (SSI0Clk)

// pin 7 MOSI PA5 (SSI0Tx)

// pin 6 TFT_CS PA3 (SSI0Fss)

// pin 5 CARD_CS unconnected

// pin 4 D/C PA6 (GPIO)

// pin 3 RESET PA7 (GPIO)

// pin 2 VCC +3.3 V

// pin 1 Gnd ground

Bard, Cuevas, Erez, Gerstlauer, Holt, Valvano, Yerraballi, Telang, Tiwari

Interface to Launchpad

Note: if your ST7735 display doesn’t look exactly like this,

see the comments in ST7735.c to find how to connect it to the TM4C123.

D/C High for Data , Low for Command

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Module Call Graph

ST7735

LCD

Low-level

main

writedata

writecommand

LCD_OutDec

LCD_OutFix

ST7735_OutChar

ST7735_OutString

Lab7Main.s

Print.s

ST7735.c

LCD.s

IO.s

IO_Init

IO_Touch

IO_Heartbeat

Bard, Cuevas, Erez, Gerstlauer, Holt, Valvano, Yerraballi, Telang, Tiwari

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LCD Programming

writecommand: Involves 6 steps performed to send 8-bit Commands to the LCD

Read SSI0_SR_R and check bit 4,
If bit 4 is high, loop back to step 1

wait for BUSY bit to be low

Clear D/C=PA6 to zero

(D/C pin configured for COMMAND)

Write the command to SSI0_DR_R
Read SSI0_SR_R and check bit 4,
If bit 4 is high loop back to step 5

(wait for BUSY bit to be low)

Bard, Cuevas, Erez, Gerstlauer, Holt, Valvano, Yerraballi, Telang, Tiwari

Think about what happens when you output multiple commands one right after another?

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LCD Programming

writedata: Involves 4 steps performed to send 8-bit data to the LCD:

Read SSI0_SR_R and check bit 1,
If bit 1 is low, loop back to step 1

(wait for TNF bit to be one)

Set D/C=PA6 to one

(D/C pin configured for DATA)

Write the 8-bit data to SSI0_DR_R

Bard, Cuevas, Erez, Gerstlauer, Holt, Valvano, Yerraballi, Telang, Tiwari

Think about what happens when you output multiple data one right after another?

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LCD – Lab7

Lab assignment

Interface LCD to TI board
Develop device driver to serve as interface between TM4C123 and ST7735R display
Write the modules you are responsible for
Test on Simulator first
Build circuit and test on real board

Bard, Cuevas, Erez, Gerstlauer, Holt, Valvano, Yerraballi, Telang, Tiwari

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