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Lecture 11 Low Power Modes & Watchdog Timers

Lecture 11 Low Power Modes & Watchdog Timers. Outline. RX210 Reset Sources RX210 Low Power Modes RX210 Watchdog Timers. RX210 Reset source. There are nine types of resets The internal state and pins are initialized by a reset When a reset is canceled, the reset exception handling starts.

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Lecture 11 Low Power Modes & Watchdog Timers

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  1. Lecture 11 Low Power Modes & Watchdog Timers

  2. Outline • RX210 Reset Sources • RX210 Low Power Modes • RX210 Watchdog Timers

  3. RX210 Reset source There are nine types of resets The internal state and pins are initialized by a reset When a reset is canceled, the reset exception handling starts

  4. RX210 Reset source (cont.) RES# Pin Reset Generated by the RES# pin When the RES# pin is driven low, all the processing in progress is aborted and the LSI enters a reset state To unfailingly reset the LSI, the RES# pin should be held low for the specified power supply stabilization time at a power-on When the RES# pin is driven high from low, the internal reset is canceled after the post-RES# cancellation wait time (tRESWT) has elapsed The CPU starts the reset exception handling Power-On Reset An internal reset generated by the power-on reset circuit If the RES# pin is in a high level state when power is supplied, a power-on reset is generated After VCC has exceeded VPOR and the specified period (power-on reset time) has elapsed, the internal reset is canceled The CPU starts the reset exception handling

  5. RX210 Reset source (cont.) The power-on reset time is a stabilization period of the external power supply and the LSI circuit After a power-on reset has been generated, the PORF flag in RSTSR0 is set to 1 The PORF flag is initialized by RES# pin reset Voltage Monitoring 0 Reset An internal reset generated by the voltage monitoring circuit If the voltage detection circuit 0 start bit (LVDAS) in option function select register 1 (OFS1) is 0 (voltage monitoring 0 reset is enabled after a reset) and VCC falls below Vdet0 The RSTSR0.LVD0RF flag becomes 1 and the voltage detection circuit generates the reset Clear the OFS1.LVDAS bit to 0 if the voltage monitoring 0 reset is to be used Release from the voltage monitoring 0 reset state occurs when VCC rises above Vdet0 and the LVD0 reset time (tLVD0) elapses The CPU starts the reset exception handling

  6. RX210 Reset source (cont.)

  7. RX210 Reset source (cont.) Voltage Monitoring 1 Reset An internal resets generated by the voltage monitoring circuit When the voltage monitoring 1 interrupt/reset enable bit (LVD1RIE) is set to 1 (enabling generation of a reset or interrupt by the voltage detection circuit) and the voltage monitoring 1 circuit mode select bit (LVD1RI) is set to 1 (selecting generation of a reset in response to detection of a low voltage) in the voltage monitoring 1 circuit control register 0 (LVD1CR0) The RSTSR0.LVD1RF flag is set to 1 and the voltage-detection circuit generates a voltage monitoring 1 reset if VCC falls to or below Vdet1 Timing for release from the reset state is selectable with the voltage monitoring 1 reset negation select bit (LVD1RN) in the LVD1CR0 register When the LVD1CR0.LVD1RN bit is 0, the CPU is released from the internal reset state and starts reset exception handling once the voltage monitoring 1 reset time (tLVD1) has elapsed after VCC has risen above Vdet1

  8. RX210 Reset source (cont.) When the LVD1CR0.LVD1RN bit is 1, the CPU is released from the internal reset state and starts reset exception handling once the voltage monitoring 1 reset time (tLVD1) has elapsed Voltage Monitoring 2 Reset An internal resets generated by the voltage monitoring circuit When the voltage monitoring 2 interrupt/reset enable bit (LVD2RIE) is set to 1 (enabling generation of a reset or interrupt by the voltage detection circuit) and the voltage monitoring 2 circuit mode select bit (LVD2RI) is set to 1 (selecting generation of a reset in response to detection of a low voltage) in voltage monitoring 2 circuit control register 0 (LVD2CR0) The RSTSR0.LVD2RF flag is set to 1 and the voltage detection circuit generates a voltage monitoring 2 reset if VCC falls to or below Vdet2 Timing for release from the reset state is selectable by setting the voltage monitoring 2/comparator A2 reset negation select bit (LVD2RN) in the LVD2CR0 register Detection levels Vdet1 and Vdet2 can be changed by settings in the voltage detection level select register (LVDLVLR)

  9. RX210 Reset source (cont.) Deep Software Standby Reset An internal reset generated when deep software standby mode is canceled by an interrupt When a deep software standby mode cancellation source is generated, a deep software standby reset is generated The deep software standby reset is canceled after tDSBY (return time after deep software standby mode cancellation) has elapsed Deep software standby mode is also canceled When tDSBYWT (wait time after deep software standby mode cancellation) has elapsed after deep software standby mode has been canceled, the internal reset is canceled The CPU starts the reset exception handling Independent Watchdog Timer Reset An internal reset generated by the independent watchdog timer Output of the reset from the independent watchdog timer can be selected by setting the IWDT reset control register (IWDTRCR) and option function select register 0 (OFS0)

  10. RX210 Reset source (cont.) A reset is generated if the independent watchdog timer underflows Or if data is written outside the refresh-permitted period When the internal reset time (tRESW2) has elapsed after the reset has been generated, the internal reset is canceled The CPU starts the reset exception handling Watchdog Timer Reset An internal reset from the watchdog timer Output of the watchdog timer reset from the independent watchdog timer can be selected by setting the WDT reset control register (WDTRCR) and option function select register 0 (OFS0) A reset is generated if the watchdog timer underflows Or if data is written outside the refresh-permitted period When the internal reset time (tRESW2) has elapsed after the reset is generated, the internal reset is canceled The CPU starts the reset exception handling

  11. RX210 Reset source (cont.) Software Reset An internal reset generated by the software reset circuit When A501h is written to SWRR, a reset is generated When the internal reset time (tRESW2) has elapsed after the reset is generated, the internal reset is canceled The CPU starts the reset exception handling By reading the CWSF flag in RSTSR1, the type of reset processing caused can be identified A power-on reset has caused the reset processing (cold start) Or a reset signal input during operation has caused the reset processing (warm start) The CWSF flag in RSTSR1 is set to 0 when a power-on reset occurs (cold start) Otherwise the flag is not set to 0 Reading RSTSR0 and RSTSR2 determines which reset was used to execute the reset exception handling

  12. RX210 Reset source (cont.) Reset Status Register 0 (RSTSR0)

  13. RX210 Reset source (cont.) Reset Status Register 1 (RSTSR1) Software Reset Register (SWRR)

  14. RX210 Reset source (cont.) Reset Status Register 2 (RSTSR2)

  15. Low Power Consumption RX210 has some functions for reducing power consumption Switching of clock signals to reduce power consumption BCLK output control Stopping modules Functions for low power consumption in normal operation Transitions to low power consumption states Reducing Power Consumption by Switching Clock Signals When the SCKCR.FCK[3:0], ICK[3:0], BCK[3:0], PCKB[3:0], and PCKD[3:0] bits are set, the clock frequency changes The CPU, DMAC, DTC, ROM, and RAM operate on the operating clock specified by the ICK[3:0] bits Peripheral modules operate on the operating clock specified by the PCKB[3:0], and PCKD[3:0] bits The flash memory interface operates on the operating clock specified by the FCK[3:0] bits The external bus operates on the operating clock specified by the BCK[3:0] bits

  16. Low Power Consumption (cont.) Module Stop Function Can be set for each on-chip peripheral module The specified module stops operating and enters the module stop state, but the CPU continues to operate independently When the MSTPmi bit (m = A to C, i = 0 to 31) in MSTPCRA to MSTPCRC is set to 1 Clearing the MSTPmi bit to 0 cancels the module stop state Allowing the module to restart operating at the end of the bus cycle The internal states of modules are retained in the stop state After a reset is canceled, all modules other than the DMAC, DTC, and RAM are placed in the module stop state Read/write access cannot be made to the registers of the module that are in the module stop state Power consumption can be reduced By selecting an appropriate operating power consumption control mode according to the operating frequency and operating voltage

  17. Low Power Consumption (cont.) Sleep mode The CPU enters sleep mode When the WAIT instruction is executed as the SBYCR.SSBY bit is 0 The CPU stops operating The contents of its internal registers are retained Other peripheral functions do not stop The WDT stops counting when sleep mode is entered Counting by the IWDT stops If a transition to sleep mode is made while the IWDT is being used in auto-start mode and the OFS0.IWDTSLCSTP bit is 1 If a transition to sleep mode is made while the IWDT is being used in register start mode and the SLCSTP bit in IWDTCSTPR is 1 Counting by the IWDT continues If a transition to sleep mode is made while the IWDT is being used in auto-start mode and the OFS0.IWDTSLCSTP bit is 0 If a transition to sleep mode is made while the IWDT is being used in register start mode and the SLCSTP bit in IWDTCSTPR is 0

  18. Low Power Consumption (cont.) To use sleep mode, make the following settings and then execute a WAIT instruction Clear the PSW.I bit of the CPU to 0 Set the interrupt destination to be used for recovery from sleep mode to the CPU Set the priority of the above interrupt to a level higher than the setting of the PSW.IPL[3:0] bits of the CPU Set the IERm.IENj bit for the interrupt to 1 For the last I/O register to which writing proceeded, read the register to confirm that the value written has been reflected Execute the WAIT instruction, which automatically sets the I bit in the PSW of the CPU to 1 Sleep mode is canceled by any interrupt And the reset signal from the RES# pin, a power-on reset, a voltage monitoring reset, or a reset caused by an IWDT underflow Canceling by an interrupt When an interrupt occurs, sleep mode is canceled and the interrupt exception handling starts

  19. Low Power Consumption (cont.) If a maskable interrupt has been masked by the CPU, sleep mode is not canceled Canceling by the RES# pin When the RES# pin is driven low, the LSI enters the reset state When the RES# pin is driven high after the reset input for a predefined time period, the CPU starts the reset exception handling Canceling by a power-on reset Sleep mode is canceled by a power-on reset Canceling by a voltage monitoring reset Sleep mode is canceled by a voltage monitoring reset from the voltage detection circuit Canceling by the independent watchdog timer reset Sleep mode is canceled by an internal reset generated by an IWDT underflow When OFS0.IWDTSTRT = 0 and OFS0.IWDTSLCSTP = 1, or OFS0.IWDTSTRT = 1 and IWDTCSTPR.SLCSTP = 1, the IWDT is stopped in sleep mode and sleep mode cannot be canceled by the independent watchdog timer reset

  20. Low Power Consumption (cont.) To switch the clock source used on return from sleep mode The clock used after return needs to be set by the sleep mode return clock source switching register (RSTCKCR) The wait control register needs to be set for each clock source When the return interrupt is generated, the clock source is automatically switched, and operation returns from sleep mode The registers related to clock source switching are automatically rewritten If a transition is made to sleep mode with the OSTDCR.OSTDE bit being 1 (oscillation stop detection function enabled), the sleep mode return clock source switching function is disabled All-module clock stop mode The CPU enters all-module clock stop mode Executing a WAIT instruction while the SBYCR.SSBY bit is 0 after setting the MSTPCRA.ACSE bit to 1 and placing modules controlled by MSTPCRA, MSTPCRB, and MSTPCRC registers in the module stop state (MSTPCRA = FFFF FF[C-F]Fh, MSTPCRB = FFFF FFFFh, MSTPCRC[31:16] = FFFFh)

  21. Low Power Consumption (cont.) Stops the bus controller, I/O ports, and all modules except for the 8-bit timers, POE, IWDT, RTC, power-on reset circuit, voltage detection circuit at the end of the current bus cycle The WDT stops counting when the mode is entered Counting by the IWDT stops If a transition to the mode is made while the IWDT is being used in auto-start mode and the OFS0.IWDTSLCSTP bit is 1 If a transition to the mode is made while the IWDT is being used in register start mode and the SLCSTP bit in IWDTCSTPR is 1 Counting by the IWDT continues If a transition to the mode is made while the IWDT is being used in auto-start mode and the OFS0.IWDTSLCSTP bit is 0 (counting by the IWDT continues through transitions to low power modes) If a transition to the mode is made while the IWDT is being used in register start mode and the SLCSTP bit in IWDTCSTPR is 0 To use all-module clock stop mode, make the following settings and then execute a WAIT instruction Clear the PSW.I bit of the CPU to 0

  22. Low Power Consumption (cont.) Set the interrupt destination to be used for recovery from all-module clock stop mode to the CPU Set the priority of the above interrupt to a level higher than the setting of the PSW.IPL[3:0] bits of the CPU Set the IERm.IENj bit for the interrupt to be used for recovery from all-module clock stop mode to 1 For the last I/O register to which writing proceeded, read the register to confirm that the value written has been reflected Execute the WAIT instruction, which automatically sets the I bit in the PSW of the CPU to 1 Release from all-module clock stop mode is initiated by an external pin interrupt (the NMI or IRQ0 to IRQ7) And a peripheral interrupt (8-bit timer, RTC alarm, RTC periodic, IWDT, voltage monitoring 1, voltage monitoring 2, or oscillator-stopped detection interrupt) And a RES# pin reset, a power-on reset, a voltage monitoring reset, or an independent watchdog timer reset The transition to the normal program execution state proceeds via exception processing for the given interrupt or reset

  23. Low Power Consumption (cont.) The interrupt will not cancel all-module clock stop mode If a maskable interrupt has been masked by the CPU or a maskable interrupt has been set up as a trigger to activate the DTC or DMAC Software standby mode The CPU enters software standby mode When a WAIT instruction is executed with the SBYCR.SSBY bit set to 1 and the DPSBYCR.DPSBY bit cleared to 0 The CPU, on-chip peripheral functions, and all the oscillator functions stop The contents of the CPU internal registers, RAM data, on-chip peripheral functions, and the states of the I/O ports are retained Whether the address bus and bus control signals are placed in the high-impedance or the output state is retained can be specified by the SBYCR.OPE bit This mode allows significant reduction in power consumption The oscillator stops in this mode Further reduction of power consumption is possible by using the FHSSBYCR register

  24. Low Power Consumption (cont.) Clear the DMAST.DMST bit of the DMAC and the DTCST.DTCST bit of the DTC to 0 before executing the WAIT instruction The WDT stops counting because the oscillator stops Counting by the IWDT stops If a transition to the mode is made while the IWDT is being used in auto-start mode and the OFS0.IWDTSLCSTP bit is 1 If a transition to the mode is made while the IWDT is being used in register start mode and the SLCSTP bit in IWDTCSTPR is 1 Counting by the IWDT continues If a transition to the mode is made while the IWDT is being used in auto-start mode and the OFS0.IWDTSLCSTP bit is 0 (counting by the IWDT continues through transitions to low power modes) If a transition to the mode is made while the IWDT is being used in register start mode and the SLCSTP bit in IWDTCSTPR is 0 When the oscillation stop detection function is enabled, software standby mode cannot be entered Execute a WAIT instruction after disabling the oscillation stop detection function (OSTDCR.OSTDE = 0)

  25. Low Power Consumption (cont.) To use software standby mode, make the following settings and then execute a WAIT instruction Clear the PSW.I bit of the CPU to 0 Set the interrupt destination to be used for recovery from software standby mode to the CPU Set the priority of the above interrupt to a level higher than the setting of the PSW.IPL[3:0] bits of the CPU Set the IERm.IENj bit for the interrupt to be used for recovery from all-module clock stop mode to 1 For the last I/O register to which writing proceeded, read the register to confirm that the value written has been reflected Execute the WAIT instruction, which automatically sets the I bit in the PSW of the CPU to 1 Release from software standby mode is initiated by an external pin interrupt (the NMI or IRQ0 to IRQ7) And peripheral interrupts (the RTC alarm, RTC periodic, IWDT, voltage monitoring 1, and voltage monitoring 2 interrupts), an RES# pin reset, a power-on reset, a voltage monitoring reset, or an independent watchdog timer reset

  26. Low Power Consumption (cont.) Release due to an interrupt When an interrupt request from among the NMI, IRQ0 to IRQ7, RTC alarm, RTC periodic, IWDT, voltage monitoring 1, and voltage monitoring 2 interrupts is generated, each of the oscillators which was operating before the transition to software standby mode resumes oscillation After the time set by the MOSCWTCR.MSTS[4:0], SOSCWTCR.SSTS[4:0], HOCOWTCR2.HSTS2[3:0], or PLLWTCR.PSTS[4:0] bits has elapsed, the chip is released from software standby mode and starts interrupt exception processing Release due to a reset on the RES# pin Clock oscillation starts as the low level is applied to the RES# pin Clock supply for the LSI starts at the same time Keep the level on the RES# pin low over the time required for oscillation of the clocks to become stable Reset exception processing starts when the high level is applied to the RES# pin Release due to a power-on reset

  27. Low Power Consumption (cont.) Release from software standby mode proceeds after a fall in the power-supply voltage leads to the generation of a power-on reset Release due to a voltage monitoring reset Release from the mode proceeds after a fall in the power-supply voltage leads to the generation of a voltage monitoring reset Release due to an independent watchdog timer reset An internal reset due to an underflow of the IWDT leads to release from software standby mode If a condition for the independent watchdog timer to stop counting applied at the time of a transition to software standby, using a reset from the independent watchdog timer to release the chip from software standby is impossible because the independent watchdog timer is stopped An example of a transition to software standby mode Made at the falling edge and canceled at the rising edge of IRQn pin An IRQn interrupt is accepted with the IRQCRi.IRQMD[1:0] bits of the ICU set to 01b (falling edge) Then the IRQCRi.IRQMD[1:0] bits are set to 10b (rising edge)

  28. Low Power Consumption (cont.)

  29. Low Power Consumption (cont.) Deep software standby mode A transition to software standby mode is made When the WAIT instruction is executed with SBYCR.SSBY set to 1 The CPU, internal peripheral modules and all functions of the oscillators are stopped Except for the RTC alarm, RTC periodic, SCL-DS, and SDA-DS The internal supply of power for these modules is stopped Power consumption is markedly reduced The contents of all the registers of the CPU and on-chip peripheral modules and the RAM data become undefined When the DPSBYCR .DEEPCUT1 bit is set to 1 The voltage detection circuit is stopped The power consumption reduction function by the power-on reset circuit is enabled Further reducing power consumption This varies the voltage detection characteristic of the power-on reset circuit

  30. Low Power Consumption (cont.) When the WDT is in use, counting also stops The oscillators and power supply to the WDT are stopped by the transition to deepsoftware standby mode Power supply to the IWDT-dedicated LOCO is stopped The IWDT is stopped and counting by the IWDT stops A transition to deep software standby mode cannot be made, but to software standby When the voltage monitoring 1 reset function (LVD1CR0.LVD1RI = 1) or voltage monitoring 2 reset function (LVD2CR0.LVD2RI = 1) is selected for the voltage detection circuit Release from deep software standby mode is initiated by any of the external pin interrupt source pins (the NMI, IRQ0- DS to IRQ7-DS, SCL-DS, or SDA-DS) And peripheral interrupts (the RTC alarm, RTC periodic, voltage monitoring 1, and voltage monitoring 2 interrupts) And an RES# pin reset, a power-on reset, or a voltage monitoring 0 reset

  31. Low Power Consumption (cont.) Canceling by external interrupt pin or internal interrupt signal Cancellation of the mode is controlled by DPSIERn (n = 0, 2) and DPSIFRn (n = 0, 2) When an interrupt is generated, the corresponding flag in DPSIFRn is set to 1 if the canceling source is enabled in DPSIERn, the mode is canceled Rising edge or falling edge can be selected by DPSIEGRn (n = 0, 2) The interrupts for which an edge can be selected are the NMI, IRQ0-DS to IRQ7-DS, SCL-DS, SDA-DS, voltage monitoring 1, and voltage monitoring 2 interrupts When a canceling source is generated, the internal power supply and LOCO clock oscillation begin The internal reset (deep software standby reset) is generated for the entire LSI A stable LOCO clock is then supplied to the entire LSI Deep software standby reset is canceled Deep software standby mode is canceled and the reset exception handling starts

  32. Low Power Consumption (cont.) When the mode is canceled by an external interrupt pin or internal interrupt signal, the RSTSR0.DPSRSTF (Deep Software Standby Reset Detect Flag) is set to 1 Release due to a reset on the RES# pin Canceled when the low level is applied to the RES# pin Reset exception processing starts when the high level is applied to the RES# pin Release due to a power-on reset Release from the mode proceeds after a fall in the power-supply voltage leads to the generation of a power-on reset Release due to a voltage monitoring 0 reset Release from the mode proceeds after a fall in the power-supply voltage leads to the generation of a voltage monitoring 0 reset In deep software standby mode, the I/O ports retain the same states from software standby mode The inside of the LSI is initialized by an internal reset generated when deep software standby mode is canceled

  33. Low Power Consumption (cont.) Upon cancellation of deep software standby mode, the reset exception handling starts immediately Whether to initialize the I/O ports or to keep retaining the I/O port states at the time of software standby mode can be selected by the DPSBYCR.IOKEEP bit. When the DPSBYCR.IOKEEP bit = 0, I/O ports are initialized by an internal reset generated when the mode is canceled When the DPSBYCR.IOKEEP bit = 1, the inside of the LSI is initialized by an internal reset generated The I/O port states remain unchanged from software standby mode even if settings of I/O ports or peripheral modules are made The retained I/O port states are released by clearing the DPSBYCR.IOKEEP bit to 0 The LSI operates according to the internal state. The DPSBYCR.IOKEEP bit is not initialized by an internal reset generated when deep software standby mode is canceled An example of a transition to deep software standby mode Made at the falling edge of the IRQn-DS pin, and canceled at the rising edge of the IRQn-DS pin

  34. Low Power Consumption (cont.) Module Stop Control Register A (MSTPCRA)

  35. Low Power Consumption (cont.) Standby Control Register (SBYCR)

  36. Low Power Consumption (cont.) Sleep Mode Return Clock Source Switching Register (RSTCKCR)

  37. Low Power Consumption (cont.) Main Clock Oscillator Wait Control Register (MOSCWTCR) Used to control the oscillation stabilization wait time of the main clock oscillator

  38. Low Power Consumption (cont.) Deep Standby Control Register (DPSBYCR)

  39. Low Power Consumption (cont.) Deep Standby Interrupt Enable Register 0 (DPSIER0)

  40. Low Power Consumption (cont.) Deep Standby Interrupt Flag Register 0 (DPSIFR0)

  41. Low Power Consumption (cont.) Deep Standby Interrupt Edge Register 0 (DPSIEGR0)

  42. Watchdog Timer The watchdog timer detects programs being out of control Has a 14-bit down-counter Can be set up to reset the chip by a reset output When counting down from the initial value causes an underflow of the counter Alternatively, generation of an interrupt request is selectable when the counter underflows The initial value for counting can be restored to the down-counter by refreshing its value The interval over which refreshing is possible can also be selected Refreshing the counter during this interval will restore its initial value for counting Attempting to refresh the counter beyond this interval leads to the output of a reset or interrupt request The refresh interval can be adjusted and used to detect the program entering runaway conditions The WDT stops counting after an underflow or an attempt at refreshing the counter beyond the allowed interval

  43. Watchdog Timer (cont.) Counting is restarted by refreshing the counter when the WDT is in register start mode When the WDT is in auto-start mode, counting is restarted automatically after output of the reset or interrupt request The WDT has two start modes: Auto-start mode Counting automatically starts after release from the reset state Register start mode Counting is started by refreshing the WDT (writing to the register) In auto-start mode, necessary settings should be made in option function select register 0 (OFS0) before release from the reset state In register start mode, necessary settings should be made in the respective registers before the counter is started by refreshing after release from the reset state Clock division ratio, refresh window start and end positions, time-out period, reset or non-maskable interrupt request output at an underflow

  44. Watchdog Timer (cont.) Block diagram of WDT

  45. Watchdog Timer (cont.) Set the WDT start mode select bit (OFS0.WDTSTRT bit) to select auto-start mode or register start mode When auto-start mode is selected (OFS0.WDTSTRT = 0) The WDT control register (WDTCR), and WDT reset control register (WDTRCR) settings are disabled The settings of option function select register 0 (OFS0) are enabled When register start mode is selected (OFS0.WDTSTRT = 1) The settings of option function select register 0 (OFS0) are disabled The WDTCR and WDTRCR settings are enabled Register start mode After canceling from the reset Set the clock division ratio, window start and end positions, and time-out period in the WDTCR register Set the reset or interrupt request output in the WDTRCR register Refresh the down-counter to start counting down from the value selected by setting the time-out period selection (WDTCR.TOPS[1:0]) bits

  46. Watchdog Timer (cont.) The WDTCR.TOPS[1:0] bits select the time-out period Period until the down-counter underflows from among 1,024, 4,096, 8,192, and 16,384 cycles Taking the divided clock specified by the WDTCR.CKS[3:0] bits as one cycle After the down-counter is refreshed, the combination of the CKS[3:0] and TOPS[1:0] bits determines the time (number of PCLK cycles) until the counter underflows The WDT does not output the reset signal As long as the program continues normal operation and the counter is refreshed in the refresh-permitted period The interval between the window start position and window end position is the refresh-permitted period and the other periods are refresh-prohibited periods The value in the counter is re-set each time the counter is refreshed and counting down continues The WDT outputs a reset signal or a non-maskable interrupt request (WUNI)

  47. Watchdog Timer (cont.) If the down-counter underflows due to a program runaway If a refresh error occurs because the counter was refreshed outside the refresh-permitted period Select reset output or interrupt request output By setting the reset interrupt request selection (WDTRCR.RSTIRQS An example of operation under the register start mode The WDT start mode select bit (OFS0.WDTSTRT) is 1 (register start mode) The reset interrupt request selection bit (WDTRCR.RSTIRQS) is 1 (reset output is enabled) The window start position selection bits (WDTCR.RPSS[1:0]) are 10b (75%) The window end position selection bits (WDTCR.RPES[1:0]) are 10b (25%)

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