MC68HC05X4 Datasheet, PDF (48/156 Pages) Motorola, Inc – HCMOS Microcontroller Unit

Freescale Semiconductor, Inc.
Resets, Interrupts and Low Power Modes
t_VDDR
V_DD
V_DD threshold (1-2V typical)
t_OXOV
OSC1
t_PORL
t_CYC
Internal
processor clock
RESET
Internal
address bus
Internal
data bus
(Internal power-on reset)
t_RL(or t_DOGL) _{(External hardware reset)}
New
7FFE 7FFE 7FFE 7FFE 7FFF _PC
Reset sequence
New New Op
PCH PCL code
Program
execution
begins
1FFE 1FFE 1FFE 1FFE
1FFF
New
PC
Reset sequence
New New Op
PCH PCL code
Program
execution
begins
Figure 1. Power-on reset and RESET
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 a stabilisation delay (t_PORL) from when the
oscillator becomes active. If the external RESET pin is low at the end of
this delay then the processor remains in the reset state until RESET
goes high. The user must ensure that the voltage on VDD has risen to a
point where the MCU can operate properly by the time t_PORL has elapsed.
If there is doubt, the external RESET pin should remain low until the
voltage on VDD has reached the specified minimum operating voltage.
This may be accomplished by connecting an external RC-circuit to this
pin to generate a power-on reset (POR). In this case, the time constant
must be great enough (at least 100 ms) to allow the oscillator circuit to
stabilise.
MC68HC05X4
Resets, Interrupts and Low Power Modes
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2-resets