MC68HC705J2 Datasheet, PDF (31/82 Pages) Freescale Semiconductor, Inc – member of the low-cost

Freescale Semiconductor, Inc.
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SECTION 5
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RESETS AND INTERRUPTS
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This section describes how resets reinitialize the MCU and how interrupts temporarily
change the normal processing sequence.
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5.1 Resets
A reset immediately stops the operation of the instruction being executed. A reset
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initializes certain control bits to known conditions and loads the program counter with
a user-defined reset vector address. The following conditions produce a reset:
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• Initial power-up (power-on reset)
• A logical zero applied to the RESET pin (external reset)
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• Timeout of the COP timer (COP reset)
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• An opcode fetch from an address not in the memory map (illegal address reset)
A reset does the following things to reinitialize the MCU:
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• Clears all implemented data direction register bits so that the corresponding
I/O pins are inputs
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• Loads the stack pointer with $FF
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• Sets the interrupt mask, inhibiting interrupts
• Clears the TOFE and RTIE bits in the timer control and status register
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• Clears the STOP latch, enabling the CPU clocks
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• Clears the WAIT latch, waking the CPU from the WAIT mode
• Loads the program counter with the user-defined reset vector
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5.1.1 Power-On Reset
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A positive transition on the V_DD pin generates a power-on reset. The power-on reset ₁₇
is strictly for power-up conditions and cannot be used to detect drops in power supply
voltage.
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RESETS AND INTERRUPTS
Rev. 2
5-1
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