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Interfacing to Microprocessors

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1 Interfacing to Microprocessors
Chapter 12

2 introduction What constitutes a “controller” will vary from application to application. It may be no more than an amplifier or a switch. It may be a complex system that may include computers and other types of processors such as data acquisition and signal processors. Most of the time, it is a microprocessors. We shall therefore focus the discussion here on microprocessors.

3 introduction Focus on microprocessors as general purpose, flexible and reconfigurable controllers and the ways sensors and actuator relate to these. Microprocessors are often called microcontrollers What is a microprocessor? What is the different between a microprocessor and a computer or a microcomputer and how a distinguishing set of features is arrived at are all difficult and subjective issues. What is a microprocessor to one is a full fledged computer to another

4 The microprocessor A microprocessor is a stand alone, self contained single chip microcomputer. It must have as a minimum: a central processing unit (CPU) nonvolatile and program memory input and output capabilities. A structure that has these can be programmed in some convenient programming language can interact with the outside world through the input/output ports.

5 The microprocessor Other important requirements:
must be relatively simple reasonably small necessarily limited in most of its features – memory, processing power and speed, addressing range and, of course in number of I/O devices it can interact with. The designer must have access to all features of the microprocessor – bus, memory, registers, all I/O ports, In short, Microprocessors are components with flexible features that the engineer can configure and program to perform task or a series of tasks.

6 The microprocessor Two limits on the tasks microprocessors can perform: The limitations of the microprocessor itself The imagination (or capabilities) of the designer.

7 The 8 bit microprocessor
We will narrow down to 8 bit microprocessors these are the most common in sensor/actuator systems they are simple and representative of all microprocessor 16 and 32 bit microprocessors exist There are a number of architectures being used. We will emphasize the Harvard architecture because of its simplicity, flexibility and popularity.

8 The architecture There are about two dozen manufacturers of microprocessors All based on a few architectures. We shall only briefly describe here one architecture – the Harvard architecture used in many microprocessors Simple and efficient The choice in smaller microprocessor Example: Microchip and Atmel microprocessors

9 The architecture Main features:
Separate busses for program memory and operand memory. Pipelined architecture Allows fetching data while another operation executes. Each cycle consists of fetching the (n+1)th instruction while executing the nth Integer arithmetic Limited instruction set

10 The architecture Bus widths vary depending on manufacturer and on the microprocessor size. Example: Figure 12.1, bus architecture for a PIC18F452 from Microchip. The instruction is 16bit Program address is 15bit wide. Data is 8bits and Operand address is 12 bits. These vary from device to device.

11 Bus architecture

12 The architecture Example, the smallest microprocessors available (PIC10FXX) are 6 pin devices Summarized in Table 12.1. The architecture for this device is shown in Figure 12.2. Here the program address bus is only 9 bits while the instruction buss is 12 bits.

13 PIC10FXX microprocessors

14 PIC10FXX microprocessors

15 The architecture Example: one of the largest, is the PIC18FXX20
Has an address bus 21 bits wide. The processor and its variants are shown in Table 12.2 Its architecture in Figure 12.3.



18 The architecture Architecture supports:
Direct addressing for the first 8 bits of address space Indirect addressing (variable pointer addressing) for all memory space. Includes a CPU with associated status bits and a set of special functions registers. I/O ports, other peripherals (such as comparators, A/D converters, PWM modules, etc.) Timers, status indications and much more,

19 The architecture All modules available to the user.
User writable registers are also provided. Microprocessors have been designed to respond to specific needs: common to find modifications that respond to these needs Example: various processors from the same family may have a different instruction sets PIC10FXX has 33 instructions PIC18FXX20 has 77 instructions ATmega128 (from Atmel) has 133 instructions.

20 The architecture Memory varies from 256 bytes to over 256 kbytes
Number of peripherals, ports, etc vary from as few as 4 to over 100 Physical size: from 6 pin to 100 pins Various chip configurations (DIP, surface mount, dies etc.)

21 Addressing 8 bit microprocessors have word length of 8 bits.
Integer data from 0 to 255 may be represented directly. To address memory, usually a longer word is needed. Most microprocessor have a 12 bit (4k) 14 (16k) or 16 bit (64k) memory address but longer address words are also used.

22 Speed Most microprocessor operate at clock speeds between 1 and 40 MHz. Since often the clock is internally divided, the instruction cycle is slower than that Typical values are up to about 10 MHz cycle clock or 0.1 s per instruction

23 Instruction set Microprocessors have a small instruction set –
sometimes no more than 2-3 dozen simple instructions. Varies from a minimum of about 30 to a maximum of about 150 instructions. These are selected to cover the common requirements of programming a device Allows one to perform almost any task that can be physically performed within the basic limitations of the device.

24 Instruction set Instructions include:
logical instructions (AND, OR, XOR, etc.) move and branching instructions (allow one to move data from and to registers and conditional and unconditional branching) bit instructions (operations on single bits in an operand) arithmetic instructions such as add and subtract, subroutine calls other instructions that have to do with the performance of the microprocessor such as reset, sleep and others. Some are bit oriented, some are byte (register) oriented, some are literal and control operations

25 Input and output Input and output is defined by the availability of pins on the package. Usually limited to less than about 100 pins (6, 8, 14, 18, 20, 28, 32, 40, 44, 64 and 100 pins are common). Two pins are used to power to the device For example, an 18 pin device can have no more than 14 I/O pins. Of these, some may be used for other purposes such as oscillators or communication

26 Input and output All microprocessor will have a number of pins available as I/O. Example, a 6 pin microprocessor may have as many as 4 I/O, a 64 pin processor can have in excess of 48 I/O pins. I/O pins are grouped into ports, each addressable as an 8 bit word (each group has up to 8 I/O pins). Different ports may have different properties and may be able to perform different functions.

27 Input and output I/O pins are tri-state enabling an I/O pin to serve as input, output or to be disconnected. Most I/O are digital but some may be configured as analog as well. I/O pins can supply or sink considerable current – usually in the range of mA. This is not sufficient to drive many actuators but it can drive low power devices directly or indirectly through switches and amplifiers.

28 Clock and timers Microprocessor must have a timing mechanism that defines the instruction cycle. This is done by an oscillator Oscillators may be internal or external. Usually and RC oscillator is used for internal oscillation A crystal is the most common way of setting the frequency externally (this requires either dedicated pins or the use of two I/O pins).

29 Clock and timers The oscillator frequency is usually divided internally to define the basic cycle time. Microprocessors have internal timers under the control of the user used for various functions requiring counting/timing At least one counter is available larger microprocessors can have 4 or more timers some are 8 bit timers and some 16 bit timers. a watchdog timer is available for the purpose of resetting the processor should it be “stuck” in an inoperative mode.

30 Clock and timers Registers Used for Execution of commands
Control over the functions of the microprocessor, Addressing Flagging Status indication

31 Memory Modern microprocessors, contain three types of memory:
program memory, in which the program is loaded, data memory (RAM), EEPROM memory Note: EEPROM not available on some very small microprocessors.

32 Memory Program memory is usually the largest
From less than 256 bytes to over 256kBytes. In most cases, flash memory which means that is rewritable at will and is nonvolatile (program is retained until rewritten or erased). Data memory (RAM) is usually quite small and may be a small fraction of the program memory Does not retain data upon removal of power. EEPROM is nonvolatile rewritable memory used mostly to write data during execution

33 Power Most microprocessor operate from 1.8V to 6V.
Some have a more limited range ( V). Based on CMOS technology: This means that: power consumption is very modest. power consumption is frequency dependent. The higher the frequency the higher the power consumed

34 Power Power is also dependent on What the processor does
Which modules are functioning at any given time. The user has considerable control over power consumption through: Choice of frequency Mode of operation Special functions such as interrupt wakeup and sleep.

35 Other functionalities
Microprocessor must have certain modules (CPU, memory and I/O) They can have many more modules Add functionality and flexibility Many microprocessors include comparators (for digitization purposes), A/D converters, Capture and Compare (CCP) modules, PWM generators Communication interfaces.

36 Other functionalities
One or two comparators are provided on many microprocessors. Depending on the microprocessors 8 or 10 bit A/D converters are provided, usually in multiple channels (4 to 16). PWM channels (up to 8) are common on some processors. Serial interfaces such as UART, SPI, two wire interface (I2C), synchronous serial and USB ports are available

37 Other functionalities
Many microprocessors provide multiple interfaces, all under the user’s control. Other functions such as analog amplifiers and even transceivers are sometimes incorporated within the chip. The I/O used for these functions are either digital I/O (for communication for example) or analog I/O (for A/D for example)

38 Programs and programmability
A microprocessor is only useful if it can be programmed. Programming languages and compilers have been designed specifically for microprocessors. The basic method of programming microprocessors is through the Assembly programming language Can be, and very often is done through use of higher level languages with C leading.

39 Programs and programmability
These are specific compilers, adapted for a class of microprocessors. They are based on a standard C compiled (such as ANSI C) and modified to produce executables that can be loaded onto the microprocessor. Most microprocessors can be programmed in circuit allowing changes to be made, or the processors to be programmed or reprogrammed after the circuit has been built.

40 Programs and programmability
Instruction sets for microprocessors are small and based on the assembly language nomenclature. Microprocessors have been designed for integer operations. Programming for control, especially sequential control is simple and logical. Floating point operations and, are either not practical or difficult and tedious. They also tend to require considerable time and should only be attempted if absolutely necessary.

41 Programs and programmability
There are both integer and floating point libraries freely available. Floating point operations are only practical on the larger microprocessors because they require much memory.

42 Examples of microprocessors
PIC10FXXX (low level, 6 pin), PIC16F62X (midrange, 18 pin), PIC18FXX20 (high level, 64 or 80 pin), Atmega128 (high level, 64 pin). A comparison of these typical processors will reveal most of the properties and capabilities of microprocessors.

43 Interfacing Issues Three basic modes:
1. Continuous dedicated monitoring of the sensor by the microprocessor 2. Polling the sensor 3. Interrupt mode

44 Continuous mode Microprocessor is dedicated for use with the sensor
Its output is monitored by the microprocessor continuously The microprocessor reads the sensor’s output at a given rate Output is then used to act

45 Poling mode Sensor operates as if the microprocessor did not exist.
Its output is monitored by the microprocessor The microprocessor reads the sensor’s output at a given rate or intervals - poling Output is then used to act

46 Interrupt mode Microprocessor is in sleep mode
Outputs of the sensor are not being processed Upon a given event, microprocessor wakes up through one of its interrupt options The sensor activates the interrupt

47 Notes: Interrupts can be timed
Interrupts can be issued by sources other than the sensor The microprocessor may be involved in other functions, separate from the sensor, such as control of an actuator Feedback from actuators may also be used to perform interrupts

48 General Interfacing Requirements
Microprocessor input interfacing requirements Microprocessor output requirements Errors introduced by microprocessors

49 Input interfacing requirements
Signal level Impedance and matching Response, frequency Signal conditioning Signal scaling Isolation Loading

50 Output interfacing requirements
Signal levels Power levels Isolation

51 Input signal levels Basic level: zero to Vdd No dual polarity signals
Must scale signals if necessary No dual polarity signals Must translate/scale as necessary Direct reading or A/D Can read voltages only AC or DC Limitations in frequency

52 Impedance P are high input impedance devices
Input current - < 1 A. Ideal for direct connection of low impedance sensors (magnetic, thermistors, thermoelectric, etc.) High impedance sensors (capacitive, pyroelectric, etc.) must be buffered Voltage followers FET amplifiers

53 Response and frequency
Most sensors are slow devices Can be interfaced directly No concern for response and frequency range Some sensors are part of oscillators Frequencies may be quite high Need to worry about proper sampling by the microprocessor

54 Response and frequency
Example: 10 mHz P, cycle time of 0.4 s. (most processor divide the clock frequency by a factor - 4 in this case) Any operation such as reading an input required n cycles, say n=5 Effective frequency: 0.5 MHz Sampling cannot be done at rates higher than 250 kHz Any sensor producing a signal above this frequency will be read erroneously

55 Response and frequency
Some solutions: Divide the sensor’s frequency Reduces sensitivity Must be done externally to the P F-V converter Introduces conversion errors Must be done externally Frequency counter at input Use output of the counter as input to mP. Expensive Faster microprocessors

56 Input signal conditioning
Offset Primarily for dc levels Can be offset up or down Usually done to remove the dc level Sometimes needed to remove negative polarity. AC signals may sometimes be coupled through capacitors to eliminate dc levels

57 Offset Example Thermistor: 500 at 20ºC
Varies from 100 to 900 for temp. between 0 and 100ºC

58 Offset At 500ºC At 0ºC At 100ºC V varies between 3.428V and 5.684V
5.684V is above the 5V operating voltage of the microprocessor

59 Offset Some solutions Remove 3.428V through an inverting amplifier
Reduce the source voltage from 12V to, say 6V. This will change the range from 1.714V to 2.842V Increase the resistor from 1000W to, say, 1500 W. This will reduce the output and will vary from 2.526V to 4.5V

60 Offset - other solutions
For ac signals Rectification Only appropriate if signal is unipolar Bi-polar signals produce negative signals Cannot be used with microprocessors

61 Offset - other solutions
Bridge connection Battery must be floating Output: 0V at 0ºC to 2.3V at 100ºC. Offset of arbitrary value can be added Done by decreasing the value of lower-left resistor 1V offset with 285.7 resistor

62 Scaling By amplification By attenuation Amplifiers are preferrable
Operational amplifiers By attenuation Resistance dividers Transformers (for ac) Amplifiers are preferrable Dividers introduce errors Transformers are noisy and big

63 Isolation Two basic methods Transformers Optical isolation

64 Loading Microprocessors load the sensor
Not an issue with low impedance sensors Must be buffered for high impedance sensors Solution: voltage followers with FET input stages An error due to loading should be taken into account

65 Output Interface Most microprocessors: 1.8 to 6V
20 to 25 mA per output pin Can power small loads directly (LEDs, small relays) Protection diodes on all outputs

66 Output Interface Large loads:
Must add circuitry to boost current, power MOSFETS are ideal for this purpose Inductive loads: must add protection against large spikes Often necessary to isolate output Very often necessary to translate voltages for output

67 Output pins MOSFETS: Driven

68 Output pins connection of loads
Sourcing current Sinking current The two are somewhat different:

69 Errors and resolution Errors introduced by the microprocessor:
Due to resolution of A/D, D/A Sampling errors These come in addition to any errors in the sensor/actuator

70 Resolution Digital systems have an inherent resolution:
LSB - least significant bit Any value smaller than the LSB cannot be represented This constitutes an error LSB is inherent in any module as well as in the CPU itself

71 Resolution of modules A/D - n bits resolution, meaning:
a 10 bit A/D, digitizing a 5V input has a resolution of: 5V/1024 = 4.88 mV The A/D can resolve down to 4.88 mV Can represent data in increments of 4.88 mV (a 14 bit A/D resolves down to 0.3 mV) For a 1V span on a sensor, this is approximately 0.5% error

72 Resolution of modules PWM (Pulse Width Modulator)
Given a clock frequency fosc, the PWM resolution is:

73 CPU errors Most microprocessors are 8 bit microprocessors
Integer arithmetics Largest value represented: 256 Roundoff errors due to this representation Special math subroutines have been developed to minimize these errors (otherwise they would be unacceptably high)

74 Sampling errors All inputs and outputs on a microprocessor are sampled. That is: Inputs are only read at intervals Outputs are only updated at intervals Intervals depend on the frequency of the clock, operation to be executed and on the software that executes it Sampling may not even be constant during operation because of the need to perform different tasks at different times Errors are due to changes in input/output between sampling to which the microprocessor is oblivious Errors are not fixed - depend among other things on how well the program is written

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