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ECE 3430 – Introduction to Microcomputer Systems University of Colorado at Colorado Springs

This lecture covers illegal opcode interrupt, SWI, WAI, and STOP HC11 instructions, COP watchdog, clock monitor, HC11 free-running timer, and timer overflow interrupt.

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ECE 3430 – Introduction to Microcomputer Systems University of Colorado at Colorado Springs

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  1. ECE 3430 – Introduction to Microcomputer SystemsUniversity of Colorado at Colorado Springs Lecture #14 Agenda Today • Illegal Opcode Interrupt • SWI, WAI, and STOP HC11 Instructions • Computer Operating Properly (COP) Watchdog • Clock Monitor • HC11 Free-Running Timer (TCNT) • Timer Overflow Interrupt ECE 3430 – Intro to Microcomputer Systems Fall 2009

  2. Interrupts Illegal Opcode Trap- If the CPU is expecting an opcode and the control unit doesn’t recognize the machine code, an illegal opcode interrupt is thrown.- This IRQ is non-maskable or always on.- If this occurs, the CPU has lost control of the program and needs to be re-initialized. This can happen if a large noise spike enters the CPU or if the programmer modified program memory directly without using the assembler.- It is good practice to reinitialize everything (STACK, etc…) or just reset the CPU.- We *should* always have the vector table setup for this: ORG $FFF8 ; initialize the illegal opcode trap vector table entry FDB IOT_ISR : IOT_ISR: SEI ; make sure no maskable interrupts occur LDS #$00FF ; re-initialize the stack LDAA #$00 ; perhaps re-initialize all registers LDAB #$00 LDX #$0000 LDY #$0000 JMP MAIN ; we don’t need RTI because we re-init the stack pointer ; ‘MAIN’ is a label for the first line of the main loop ECE 3430 – Intro to Microcomputer Systems Fall 2009

  3. Interrupts Interrupt Overhead- Interrupts take 12 clock cycles to initialize and get ready to execute the ISR. RTI takes 12 clock cycles to return the program to normal operation.- The overhead should be measured to determine if an IRQ should be used. If the interrupts occur too close together, some may be lost!- If something tries to interrupt every 20 clock cycles, every other interrupt will be “lost”. The HC11 cannot acknowledge interrupts this quickly. ECE 3430 – Intro to Microcomputer Systems Fall 2009

  4. Interrupts Software Interrupt Instruction (SWI)- This is an instruction that we can use to force an interrupt (SWI). This is an example of a software-generated interrupt.- This is a non-maskable interrupt (makes sense since we explicitly call it—if we wanted to mask it, we wouldn’t call it in the first place).- Used for debugging of code (software debuggers would use it to set breakpoints). ECE 3430 – Intro to Microcomputer Systems Fall 2009

  5. Interrupts Example) Insert a breakpoint in the main loop that will stop the normal execution of the program until PA0 is a logic 0. Perhaps an active-low pushbutton is interfaced to PA0. This push would be the “resume” button. ORG $FFF6 ; initialize the SWI vector table entry FDB SWI_ISR ORG $FFFE ; initialize reset vector table entry FDB $E000 ORG $E000 ; begin code at top of EEPROM SEI ; disable maskable interrupts LDS #$FF ; initialize stack pointer MAIN: … ; do something SWI ; at this point, stop program … ; do something DONE: BRA DONE ; done * * SWI service routine * SWI_ISR: LDAA PORTA ; now wait until a ‘0’ is present on PA0 ANDA #%00000001 BNE SWI_IRQ RTI ECE 3430 – Intro to Microcomputer Systems Fall 2009

  6. Interrupts Semi-Low Power Mode (WAI) - When not using the CPU, we might like to stop operation to save power. Maybe we need to synchronize software execution with the occurrence of an interrupt.- “Wait for Interrupt” (WAI) is an instruction that can do this. - Puts the microcontroller in a semi-low power mode (no opcodes read but keeps clock oscillator running). - The programming model registers/accumulators are stored to the stack (just like when an interrupt occurs). This is done in anticipation of the interrupt that will occur to satisfy the wait. - While waiting, the CPU is issuing “dummy reads” to RAM—when the clock oscillator is running, the CPU has to do *something*. - The CPU only resumes when a non-maskable interrupt, XIRQ, or RESET occurs. - When the interrupt occurs, the CPU fetches the ISR vector and dives right into the ISR (lower latency than normal since the WAI pushed the programming model ahead of time). ECE 3430 – Intro to Microcomputer Systems Fall 2009

  7. Interrupts Low power mode (STOP)- True low power mode can be accomplished using the STOP instruction.- This disables all clocks if the S-bit in the CCR is 0: S=1, STOP disabled, instruction is ignored (interpreted as NOP) S=0, STOP enabled- Since the clocks are stopped, the CPU does absolutely nothing and consumes very little power. - To get out of a STOP condition one of the following must occur (other interrupts are ignored): IRQ (I-bit must be clear in CCR to resume from STOP) XIRQ (if X-bit is set in CCR, ISR is skipped and execution cont. after STOP) RESET - NOTE: If you try this in the lab, place a NOP instruction prior to the STOP (counters a flaw in some HC11 mask sets [but likely all these faulty HC11s are long gone now]): … NOP STOP ECE 3430 – Intro to Microcomputer Systems Fall 2009

  8. Resets Computer Operating Properly (COP) Watchdog Timer Reset- An independent free-running counter that will reset the CPU if it ever overflows (internal reset).Theory- Under normal operation, code is written to clear the counter periodically. - If software stops running for some reason, the counter will not be reset by the program and will overflow. This causes a RESET and the program will begin executing at the beginning. - This provides auto-recovery from disaster situations (a requirement for space craft for example).- To enable/disable the COP timer, we alter the ‘NOCOP’ bit in the ‘CONFIG’ register. NOCOP = 0, enabled (RESET condition) NOCOP = 1, disabled- The time it takes for the COP counter to overflow is called the “Timeout Period”. This can be programmed to 4 different values using the CR1 and CR0 bits in the OPTION register.CR1CR0E/215 0 0 1 0 1 4 1 0 16 1 1 64 ECE 3430 – Intro to Microcomputer Systems Fall 2009

  9. Resets Computer Operating Properly (COP) Watchdog Timer ResetExample) CR1=0 & CR0=1 with XTAL = 8 MHz. What is the COP Timer Overflow Period? E-clock = XTAL/4 = 2MHz2MHz = 61.035 Hz  61.035 = 15.259 Hz (215) 4 Tcop = 1/(15.259 Hz) = 65.536 ms ECE 3430 – Intro to Microcomputer Systems Fall 2009

  10. Resets Computer Operating Properly (COP) Watchdog Timer ResetTo reset the COP timer (when enabled): 1) write $55 to COPRST, followed by 2) write $AA to COPRSTTIMER AUTO-RESETSTo initialize the interrupt vector table for the COP reset, we insert the beginning addressof our ISR just like all the others: ORG $FFFA ; initialize COP reset vector FDB $E000 ORG $FFFE ; initialize RESET vector FDB $E000 ECE 3430 – Intro to Microcomputer Systems Fall 2009

  11. Resets Clock Monitor Reset- An RC circuit is present in the HC11.- The capacitor is charged up with the clock.- If the clock stops, the stored charge will decay and trip a RESET (internal reset).- Roughly: Eclk < 10kHz, trips reset Eclk > 200kHz, prevents reset Reset is tripped ECE 3430 – Intro to Microcomputer Systems Fall 2009

  12. Resets Clock Monitor Reset- The CME bit in the OPTION register will enable/disable the clock monitor reset interrupt CME = 0, disabled (Reset state) CME = 1, enabled Remember: = RCVout = Vin · e-(t/) Volts (V) Time (t) ECE 3430 – Intro to Microcomputer Systems Fall 2009

  13. Timers Timers- We want separate timing circuitry that will run independent of our program. This ensures more precise timing. Our program run times can be unpredictable due to interrupts! For example, a DELAY subroutine or loop may take longer to complete if it is interrupted by an interrupt.- The HC11 (D3) provides a timer sub-system (on-chip peripheral devices): 1) 16-bit Main Timer (TCNT) 2) Input Capture (4 channels) 3) Output Compare (5 channels) 4) Real Time Interrupts 5) COP Timer System 6) Pulse Accumulator ECE 3430 – Intro to Microcomputer Systems Fall 2009

  14. Main Timer Free Running Main Timer (TCNT) / Timer Overflow Interrupt- A 16-bit counter clocked by a version of E-clock.- Has pre-scalar to slow count down.- When overflow occurs ($FFFF  $0000), an interrupt can be triggered.- On RESET, the counter is cleared to $0000.- This counter is READ ONLY.- When overflow occurs, the “Timer Overflow Flag” (TOF) is set.- The “TOF” bit is located in the TFLG2 register ($0025). TOF = 1, overflow has occurred, to clear flag write a ‘1’ to TOF TOF = 0, waiting for overflow- The interrupt “Timer Overflow IRQ” is triggered on overflow: Global Enable = I-bit in CCR Local Enable = TOI bit in the TMSK2 register Interrupt when logic 0  TOI Pre-Scalardivide by1, 4, 8, or 16 Main Timer(TCNT) 16-bit  IRQ   E-clock   TOF TCNT High TCNT Low ECE 3430 – Intro to Microcomputer Systems Fall 2009

  15. Main Timer Free Running Main Timer (TCNT) / Timer Overflow Interrupt - The “Timer Overflow Interrupt” (TOI) flag is located in the TMSK2 register ($0024). TOI = 1, Enabled TOI = 0, Disabled (Reset state)- The “Timer Pre-Scalars” (PR1, PR0) are also in the TMSK2 register ($0024).PR1PR0Pre-ScalarOverflow Period (Eclk = 2 MHz) 0 0 1 32.77 ms 0 1 4 131.1 ms 1 0 8 262.1 ms 1 1 16 524.3 ms NOTE: The Pre-Scalars can only be changed ONCE, within the first 64 E-clock cycles. Interrupt when logic 0  TOI Pre-Scalardivide by1, 4, 8, or 16 Main Timer(TCNT) 16-bit  IRQ   E-clock   TOF TCNT High TCNT Low ECE 3430 – Intro to Microcomputer Systems Fall 2009

  16. Main Timer Free Running Main Timer (TCNT) / Timer Overflow Interrupt- When the “Timer Overflow” interrupt occurs (when enabled), the TOF flag is set. - It is the responsibility of the ISR to clear the TOF flag before leaving the ISR. If the ISR fails to clear the flag, the interrupt will not go away—as soon as you execute RTI, the ISR will enter again. FLAG REGISTERS ARE DESIGNED TO ONLY ALLOW SOFTWARE TO CLEAR BITSIN THEM! ONLY HARDWARE CAN SET THE FLAGS! To clear bits in flag registers, software writes a one to that bit location. Writing a zero has no effect! To clear the TOF flag in the ISR, we would do the following (don’t use logic masks): LDAA #%10000000 ; TOF bit is the most-significant bit in TFLG2 register STAA TFLG2 Examples of flag registers: TFLG1, TFLG2 ECE 3430 – Intro to Microcomputer Systems Fall 2009

  17. Main Timer Free Running Main Timer (TCNT) / Timer Overflow Interrupt Example) Use TCNT to pulse PA4 every 131.1 ms. * [PR1,PR0] = [01] = 4 = 131.1 ms TMSK2 EQU $24 TFLG2 EQU $25 PORTA EQU $00 ORG $FFDE ; initialize “timer overflow” vector table entry FDB TCNT_ISR ORG $FFFE ; initialize “reset” vector table entry FDB $E000 ORG $E000 SEI ; disable maskable interrupts during setup LDS #$00FF ; initialize stack pointer LDAA #%10000000 ; clear the timer overflow flag by writing a ‘1’ STAA TFLG2 LDAA TMSK2 ; enable local interrupt enable (TOI) by writing a ‘1’ ORAA #%10000001 ; set bit 1 to 0 and bit 0 to 1 (pre-scaler) ANDA #%11111101 STAA TMSK2 CLI ; enable maskable interrupts DONE: BRA DONE ECE 3430 – Intro to Microcomputer Systems Fall 2009

  18. Main Timer Free Running Main Timer (TCNT) * * ISR for timer overflow interrupt. * Pulse PA4 (1 -> 0) and clear the TOF flag. * TCNT_ISR: LDAA PORTA ; sample port A ORAA #%00010000 ; force PA4 high (in ACCA), leave other bits alone STAA PORTA ; force PA4 high on port A ANDA #%11101111 ; force PA4 low (in ACCA), leave other bits alone STAA PORTA ; force PA4 low on port A LDAA #%10000000 ; force TOF (bit 7) low by writing a ‘1’, ‘0’ doesn’t effect other bits STAA TFLG2 RTI ; always call RTI to leave interrupt service routines 131.1ms ISR PA4 How accurate would you expect this waveform to be? ECE 3430 – Intro to Microcomputer Systems Fall 2009

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