Introduction to ARM Micro-controllers

1. Introduction toIntroduction to
ARM Micro-controllersARM Micro-controllers
Ravikumar Tiwari
Assistant Professor
Dept. of Electronics, GHRCE
ravikumar.tiwari@raisoni.net
www.facebook.com/rravik
www.twitter.com/Ravi6096

2. ARM family of micro-controllersARM family of micro-controllers
“ARM” architecture: designed by Advance
RISC Machines Ltd., U.K. (www.arm.com)
Designs the ARM range of RISC processor
cores Many versions exist with backward
compatibility.
ARM does not fabricate silicon itself
ARM7TDMI is one popular ARM version.
Several manufacturers.

3. ARM Powered Products

4. ARM architecture basicsARM architecture basics
 Reduced Instruction Set Computer (RISC).
 32 bit architecture
 Optional 16 bit THUMB mode in ARM7TDMI
 Pipeline architecture
 Data-processing operations only on registers.
 “Accumulator” register not provided and not required.
 32 bit arithmetic operations in 1 instruction (except
division).
 Various modes of operation: user, supervisor …
 Exceptions such as interrupt, memory abort …
 Other special features to support Operating Systems.

5. ARM7TDMI architectureARM7TDMI architecture
One of the most popular ARM variant architectures.
Offers very good price/performance ratio.
2 Operating States:
◦ 32 bit ARM
◦ 16 bit THUMB.
ARM state offers very high performance (speed).
THUMB state offers tight code density (size).
Operating State can be changed by software.
Instruction length is 32 bit (ARM) or 16 bit (THUMB)

6. Pipelining
The ARM7TDMI core uses a pipeline to increase the speed of the
flow of instructions to the processor. This enables several operations
to take place simultaneously. The processing and memory systems
to operate continuously
A three-stage pipeline is used, so instructions are executed in three
stages:.

7. Fig. ARM7TDMI processor block
diagram

8. Fig. ARM7TDMI main
processor logic

9. Fig. ARM7TDMI processor functional diagram

10. ARM7TDMI modesARM7TDMI modes
7 modes of operation:
User (usr): normal program execution mode.
FIQ (fiq): to support DMA kind of operations.
IRQ (irq): for general purpose interrupt
handling.
Supervisor (svc): protected mode for OS.
Abort (abt): to handle memory access abort.
System (sys): Privileged user mode for OS.
Undefined (und): Entered when undefined
instruction is executed.

11. ARM7TDMI registersARM7TDMI registers
Total 31 registers – each 32 bit.
At any time, 16 general registers and 1 or 2 status
registers are visible in ARM state.
General registers are named R0 to R15.
R15 is same as Program Counter (PC).
R14 is same as Link Register (LR). When a sub-routine
is called, return address is automatically saved in LR.
R13 is often used as Stack Pointer (SP).

12. ARM7TDMI registers …ARM7TDMI registers …
CPSR (Current Program Status Register) contains
condition code flags (like Z, C) and current mode
bits.
SPSR (Saved Program Status Register) holds saved
copy of previous CPSR.
A subset of ARM registers is visible in THUMB
state.
Only R0 to R7 and PC, LR, SP are directly visible in
THUMB state.

13. ThumbThumb
 Thumb is a 16-bit instruction set
◦ Optimised for code density from C code (~65% of ARM code size)
◦ Improved performance from narrow memory
◦ Subset of the functionality of the ARM instruction set
 Core has additional execution state - Thumb
◦ Switch between ARM and Thumb using BX instruction
015
31 0
ADDS r2,r2,#1
ADD r2,#1
32-bit ARM Instruction
16-bit Thumb Instruction
For most instructions generated by compiler:
 Conditional execution is not used
 Source and destination registers identical
 Only Low registers used
 Constants are of limited size
 Inline barrel shifter not used

14. ARM / THUMB advantageARM / THUMB advantage
Many ARM operations can be done in 1 THUMB
instruction; some need 2 or more THUMB
instructions.
Using THUMB instructions results in 30-40%
reduction in code size.
Using ARM instructions provides faster performance.
Intermixing of ARM and THUMB code results in
optimum solution in terms of code size and speed.

15. ARM Instruction SetARM Instruction Set
All instructions are conditionally executed. Conditions
can be “if Z flag set”, “if C flag clear” etc. “Always” is
a valid condition.
Many instructions are of the form: “ADD R1,R2,R3” –
add contents of R2 with contents of R3 and store result
in R1.
Instructions to add, subtract, multiply 8/16/32 bit
numbers.
Instructions for logical operations AND, OR, EXOR…

16. ARM Instruction Set …ARM Instruction Set …
Instructions for comparison of 32 bit numbers.
MOV instructions to move contents of
registers and constants.
Immediate addressing mode supported, but not
all values of constants are legal.
Special Multiply-Accumulate instruction.
Load / store instructions to move data between
registers and memory.

17. ARM Instruction Set …ARM Instruction Set …
Load / store instructions support various addressing
modes like indexed, pre/post increment/decrement of
base, write-back of base and so on.
Special SWAP instruction to exchange contents of a
register and a memory location.
Branch (B) instruction to transfer control.
Branch and Link (BL) instruction to call a sub-routine.
BX instruction to branch and change state (ARM /
THUMB).

18. Applications of ARM Micro-controllersApplications of ARM Micro-controllers
Any application desiring 32 bit performance.
3.3 V (or even lower voltage) ARM micro-controllers
– suitable for battery operation.
Applications requiring faster performance at lower
power consumption.
Applications requiring higher performance at not-so-
high prices.

19. Some ARM7TDMI family membersSome ARM7TDMI family members
LPC21xx family (Philips).
ADuC7026 (Analog Devices).
AT91RM42800A (Atmel).

20. Future of ARMFuture of ARM
ARM is already most popular among 32 bit micro-
controllers.
Competes equally well with 16 bit micro-controllers,
due to THUMB architecture.
Competes with 8 bit micro-controllers due to better
price/performance ratio.
Newer ARM architectures include DSP capabilities to
compete with DSP processors.
ARM is likely to stay popular for long time.

21. Links for more information:Links for more information:
www.arm.com
www.semiconductors.philips.com
www.analogdevices.com
www.atmel.com
www.spjsystems.com/armboards.htm
www.spjsystems.com/scarm.htm
Thanks for your time!

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