MC68HC05E0 Datasheet, PDF (38/96 Pages) Motorola, Inc – High-density complementary metal oxide semiconductor (HCMOS) microcontroller unit

4.1.1 Power-on Reset
A power-on reset occurs when a positive transition is detected on VDD. The power-on reset
function is strictly for power turn-on conditions and should not be used to detect drops in the power
supply voltage. The power-on circuitry provides for a 4064 tCYC delay from the time that the
oscillator becomes active. If the external RESET pin is low at the end of the 4064 cycle time-out,
the processor remains in the reset state until RESET goes high. The user must ensure that the
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voltage on VDD has risen to a point where the MCU can operate properly by the time the 4064
cycles have elapsed. If there is doubt, the external RESET pin should remain low until the voltage
on VDD has reached the specified minimum operating voltage.
4.1.2 RESET Pin
When the oscillator is running in a stable state, the MCU is reset when a logic zero is applied to
the RESET input for a minimum period of one and one-half machine cycles (tCYC). This pin
contains an internal Schmitt Trigger as part of its input to improve noise immunity.
4.2
Interrupts
The MCU can be interrupted by seven different sources: six maskable hardware interrupts and
one non-maskable software interrupt:
• External signal on the INTX pin,
• Real Time Interrupt,
• Port C Wake-up,
• Timer A,
• Timer B,
• Serial Interface,
• Software Interrupt instruction (SWI).
Interrupts cause the processor to save the register contents on the stack and to set the interrupt
mask (I bit) to prevent additional interrupts. The RTI instruction (ReTurn from Interrupt) causes the
register contents to be recovered from the stack and normal processing to resume.
Unlike reset, hardware interrupts do not cause the current instruction execution to be halted, but
are considered pending until the current instruction is complete.
Note: The current instruction is the one already fetched and being operated on.
MOTOROLA
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RESETS, INTERRUPTS AND LOW POWER MODES
MC68HC05E0