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Neutron Monitor Workshop 3(A): Microcontroller Principles

and the Demo Board

Mahidol University

January 5, 2010

Paul EvensonUniversity of DelawareBartol Research Institute

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Workshop Series Idea

• Introduce students to technical aspects of neutron monitor operation

• Rotating workshop series that will repeat every two years at a two per year rate

• Independent enough so students can join at any point

• Accommodate wide skill range with an emphasis on “hands on” experience and individual discussion

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Workshop Series Plan

1. Detector operation A. Detector Physics and Analog ElectronicsB. Art and Science of Soldering

2. Digital CircuitsA. Principles of Digital LogicB. Neutron Monitor Digital Electronics

3. MicrocontrollersA. Principles and the Demo BoardB. Next Generation Readout Board

4. Real time data acquisition A. Principles of Telemetry and Data AcquisitionB. Data Conversion and Manipulation with Visual Basic

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Plan For Today

• Lecture with Demonstrations– Discuss Microcontrollers– Explain DEMO8 Board– Explain Code Warrior– Demonstrate Simple Program

• Individual work on programs– Several CodeWarrior stations– Only one DEMO8 board– Take turns to test

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Microcomputer vs. Microcontroller

• Microcomputer– Very fast (GHz)– Powerful

Instructions– Manage large

memory– High speed general

data pathways– Often a “chip set”– Expensive ($100’s)

• Microcontroller– Modest speed

(10Mhz)– Simple instructions– Small memory

(mainly ROM)– I/O to communicate

with logic– Typically single chip– Cheap ($5)

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FPGA vs. Microcontroller

• FPGA– Very fast (GHz)– No fixed

instructions– No fixed memory– No structured I/O– Often has a

microcomputer on same chip

– Expensive ($100’s)

• Microcontroller– Modest speed

(10Mhz)– Simple instructions– Small memory

(mainly ROM)– I/O to communicate

with logic– Typically single chip– Cheap ($5)

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Essence of a Microcontroller:MC68HC908AP64

• All pins either sense or produce (and most can do either) digital logic levels under control of the internal program

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Behind This Innocent Exterior Lurks A Complex Mind!

• In addition to the straightforward input output lines there are functions to construct timing patterns and to communicate in standard ways with other devices

• All of this is under the control of the program stored in ROM (Read Only Memory)

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Programming (I)

• Computer– Operating system

converts “user friendly” code into internal instructions

– Operating system loads the program into RAM (supplemented by virtual memory on disk)

– Operating system manages data flow into and out of the program

– Operating system manages execution and terminates runaway programs

• Microcontroller– Separate system on

a computer (Code Warrior) converts “user friendly” code into internal instructions

– Hardware device (MON08 Multilink) stores the program in ROM on the chip

– All I/O is under direct program control

– Limited protection against program faults

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Programming (II)

• Code Warrior in fact has the capability to interpret high level languages (C++)

• Common functions can be captured in subroutines and linked into a program

• I will however stick to the simplest kind of programming (assembly language) where each line of code translates to a specific machine instruction

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“Stored Program” Computer

• Memory– Series of locations each with an address– MC68HC908AP64 uses 8-bit bytes– Contain binary bit patterns– Program and data are intermixed

• Program Counter– Points to the memory location containing

the next instruction• Registers

– Special memory locations implicitly referenced by instructions

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Computer Instruction Cycle

• Fetch the Instruction Pointed to by Program Counter– Instructions can be several bytes long– First byte of the instruction gives length

• Execute the Instruction– Modify memory and/or registers as specified

by the instruction• Set Condition Codes

– Usually indicate whether a result is +/0/-• Update PC to location of next instruction

– Can be as simple as n+1– Often based on instruction and condition

codes

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Some Registers are Part of the Memory Map, and are Accessed Like Data

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CPU Registers are Not Part of the Memory Map; In a Microcontroller They Are Very Simple

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Accumulator and Index Register

• The accumulator is a general-purpose 8-bit register.– The CPU uses the accumulator to hold operands and the

results of arithmetic/logic operations.

• The 16-bit index register allows indexed addressing of a 64-Kbyte memory space. H is the upper byte of the index register, and X is the lower byte. H:X is the concatenated 16-bit index register.– In the indexed addressing modes, the CPU uses the

contents of the index register to determine the conditional address of the operand.

– The index register can serve also as a temporary data storage location.

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Program Counter

• The program counter is a 16-bit register that contains the address of the next instruction or operand to be fetched.– Normally, the program counter automatically

increments to the next sequential memory location every time an instruction or operand is fetched.

• Jump, branch, and interrupt operations load the program counter with an address other than that of the next sequential location.

• During reset, the program counter is loaded with the reset vector address located at $FFFE and $FFFF.– The vector address is the address of the first

instruction to be executed after exiting the reset state.

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The Stack• The “stack” is a group of memory locations

specially arranged to allow quick storage and retrieval of temporary data.– The latest entry placed on the stack is the first to be

removed– Primarily this is used to store return addresses for

multiple layers of subroutines• The stack pointer is a 16-bit register that

contains the address of the next location on the stack. – During a reset, the stack pointer is preset to $00FF. – The reset stack pointer (RSP) instruction sets the least

significant byte to $FF and does not affect the most significant byte.

• The stack pointer decrements as data are pushed onto the stack and increments as data are pulled from the stack.

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Condition Code Register

• The 8-bit condition code register contains the interrupt mask and five flags that indicate the results of the instruction just executed. Bits 6 and 5 are set permanently to logic 1. The five logic flags are:

1. The CPU sets the overflow flag when a two's complement overflow occurs. The signed branch instructions BGT, BGE, BLE, and BLT use the overflow flag.

2. The CPU sets the half-carry flag when a carry occurs between accumulator bits 3 and 4 during an add-without-carry (ADD) or add-with-carry (ADC) operation. The half-carry flag is required for binary-coded decimal (BCD) arithmetic operations. The DAA instruction uses the states of the H and C flags to determine the appropriate correction factor.

3. The CPU sets the negative flag when an arithmetic operation, logic operation, or data manipulation produces a negative result, setting bit 7 of the result.

4. The CPU sets the zero flag when an arithmetic operation, logic operation, or data manipulation produces a result of $00.

5. The CPU sets the carry/borrow flag when an addition operation produces a carry out of bit 7 of theaccumulator or when a subtraction operation requires a borrow. Some instructions — such as bit test and branch, shift, and rotate — also clear or set the carry/borrow flag.

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Typical Logic Instruction

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Typical Branch Instruction

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Subroutine Call and Return

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DEMO908AP64 Board

• Carries a sample MC68HC908AP64

• Several simple peripherals

• Connection to other circuits

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Code Warrior and the MON08 USB Multilink Interface

• The development environment CodeWarrior and the USB Multilink together allow construction and testing of programs for the microcontroller.

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Functions of the Development System

• Assembler/Compiler– Manages user code– Adds processor description

• Linker– Combines several compiled programs– Adds further processor information

• Loader– Transfers the program to the chip

• Debugger– Allows step by step execution of code– Not all that useful for realtime stuff

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Demonstration

• First I will demonstrate a small program that flashes patterns of lights on the Demo board

• Then we can either jointly try to make it do something different, or you can have your own copies to use CodeWarrior on different machines.

• To test the programs we must use the copy of Code Warrior linked to the Demo board