Digital Electronics EEE3017W

1
Microcontroller Reliability Issues

• Often microcontroller systems are used in
  environments where high reliability is required
• This requires the hardware and software systems
  to be of high quality
• The applications must also be able to recover
  from possible error conditions gracefully

2
System Malfunctions

• Even if the hardware and software systems are
  well designed, they might malfunction
• It is important to note that not all malfunctions
  are of equal magnitude
• There are two main factors to consider when
  rating a malfunction
• How often does the malfunction occur
• How serious is it
• The seriousness of a problem must justify the
  cost in fixing it

3
GT16s Reliability Features

• The GT16 has several features which can be used
  to recover from errors
• These protect against
• Infinite loop lock up
• Power supply brown out
• One way of recovering from these errors is to
  reset the microcontroller

4
GT16s Reliability Features

• The GT16 has several modules which can be used to
  improve system reliability
• These include the
• Computer Operating Properly (COP) module
• Low Voltage Detection Module
• Illegal Address Reset
• Illegal Opcode Reset

5
Computer Operating Properly Unit

• The COP module is used to prevent runaway code
  and infinite looping
• It consists of a large counter connected to the
  systems clock
• If the counter overflows a reset is triggered
• The program should reset the counter often so
  that it does not overflow
• Therefore the COP module should not cause a
  system reset under normal operation
• The COP module is also known as the watch dog
  timer

6
System Options Register

• This is known as the SOPT register
• It is used to setup the COP module and to enable
  Background debugging
• There are two possible count values
• COPT 1 COP period is 218 cycles
• COPT 0 COP period is 213 cycles
• Cycles are from the bus clock

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Example

• What is the maximum time between COP resets if
  the bus frequency is 18.87 MHz and the COPT bit
  is HIGH?

8
Solution
9
COP Module Resetting

• Resetting the COP counter is achieved by writing
  data to a register known as the SRS
• Any write to this address causes the timing
  period to start at zero
• The COP module can also be disabled by clearing
  the COPD (COP Disable) bit in the SOPT register
• Do not place the COP reset in an interrupt
  handler
• The main program might crash, interrupt handling
  may work but the COP will still be reset
  periodically and will not provide any protection

10
Low Voltage Detect Module

• A microcontroller requires a fairly stable
  voltage supply to operate correctly
• If the voltage is unstable for some reason, a
  malfunction may occur
• This could be caused by the microcontroller
  malfunctioning or the surrounding circuitry
  causing false inputs to the controller
• If the microcontroller still functions properly
  but the surrounding circuits shutdown and power
  up again this could cause very serious system
  failure

11
Low Voltage Detection

• LVD is used to help protect against these types
  of faults
• It is also known as a brown out protection system
• A brown out is a temporary power dip
• If the supply voltage is dipping but not far
  enough to cause a system malfunction, then GT16
  can inform your program by means of the Low
  Voltage Warning System (LVW)
• These modules are controlled by the System Power
  Management Status and Control Registers

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System Power Management Status and Control
Registers

• This is known as the SPMSC1 register

13
System Power Management Status and Control
Registers

• This is known as the SPMSC2 register

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Illegal Address Reset

• Often program crashes result from it trying to
  refer to an illegal address at some point
• As an example lets say that you implement a RTS
  instruction from an interrupt
• This loads the top two bytes in the stack into
  the PC
• In the case of an interrupt the top two bytes are
  the CCR and the accumulator
• These would be loaded into the PC and interpreted
  as an address
• This could cause maloperation

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Illegal Opcode Reset

• Each instruction in the HC08 is 8-bits wide
• This means that there should be 256 possible
  opcodes
• Not all of these opcodes are implemented
• If the program attempts to execute an
  unimplemented opcode a system reset will occur

16
System Reset Status Register

• This is known as the SRS register
• A logic 1 indicates the cause of the latest
  system reset
• It is also used to reset the COP module

17
Low Power Microcontroller Operation

• Microcontrollers are often used in applications
  where power conservation is important
• These include
• Portable Applications
• Standby Systems
• Remote Applications
• It is important to conserve power for
  environmental reasons
• In all of these applications, designers need to
  manage how much power their systems consume

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Low Power Microcontroller Operation

• The faster electronic circuitry runs, the more
  power it needs
• The faster you clock your microcontroller the
  higher current it will draw
• The relationship between current and clock speed
  is quite linear
• For this reason your clock speed should be
  matched to the task that it is to perform
• Some systems have software controller clock
  management systems, however the GT16 does not

19
GT16 Low Power Modes

• Another way to save power is to turn the
  processor off when it is not being used
• The GT16 has two low power modes
• Wait
• Stop
• These modes differ in the amount of circuitry
  they shut down
• Using these modes we are able to put the system
  into low power mode with out causing a system
  reset
• The wake up signal can be provided by an external
  signal or by one of the on board peripherals

20
Wait Mode

• In wait mode the CPUs clock is stopped but
  peripheral clocks are enabled
• This means that the CPUs clock draws very little
  current
• The peripherals can therefore wake up the CPU
• Wait mode is entered by using the WAIT instruction

21
Peripherals in Wait Mode

• In wait mode these peripherals have the following
  status
• IRQ This pin may wake the CPU by causing an
  interrupt
• ADC This maybe enabled or disabled. The ADC
  draws a lot of power therefore if it is not used
  disable it. If it is used it will wake up the CPU
  when it causes an interrupt
• TIMER It also consumes a lot of power due to its
  clock. If it is not used then disable it. A timer
  interrupt will wake the CPU

22
Stop Mode

• If the wait mode consumes too much power then use
  STOP mode
• This mode causes the crystal oscillator to shut
  down which stops all clocks
• Practically the only power consumed by the device
  is leakage current
• There are fewer wake options in this mode
• It is entered by using the STOP instruction

23
Peripherals in Stop Mode

• The status of the peripherals in stop mode is as
  follows
• IRQ This pin doesnt need a clock and therefore
  can wake up the device
• ADC No wakeup possible
• TIMER No wakeup possible

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Notes for Low Power Mode

• The processor is not the only thing that draws
  current in a microcontroller
• Port pins also do therefore check that no pins
  are sourcing or sinking current in low power mode
• Check that all inputs are held either high or low
  especially for the ADC pins