HT46R0664 datasheet(36/92 Pages) HOLTEK

36 / 92 page

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Rev. 1.00

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HT46R0664

Enhanced A/D+LCD Type 8-Bit OTP MCU

Reset and Initialisation

A reset function is a fundamental part of any microcontroller ensuring that the device can be set to

some predetermined condition irrespective of outside parameters. The most important reset condition

is after power is first applied to the microcontroller. In this case, internal circuitry will ensure that

the microcontroller, after a short delay, will be in a well defined state and ready to execute the first

program instruction. After this power-on reset, certain important internal registers will be set to

defined states before the program commences.

Another type of reset is when the Watchdog Timer overflows and resets the microcontroller. All

types of reset operations result in different register conditions being setup. Another reset exists in the

form of a Low Voltage Reset, LVR, where a full reset, is implemented in situations where the power

supply voltage falls below a certain threshold.

Reset Functions

There are four ways in which a microcontroller reset can occur, through events occurring both

internally and externally:

● Power-on Reset

The most fundamental and unavoidable reset is the one that occurs after power is first applied to

the microcontroller. As well as ensuring that the Program Memory begins execution from the first

memory address, a power-on reset also ensures that certain other registers are preset to known

conditions. All the I/O port and port control registers will power up in a high condition ensuring

that all pins will be first set to inputs.

The microcontroller has an internal RC reset function, due to unstable power on conditions.

This time delay created by the RC network ensures the state of the POR remains low for an

extended period while the power supply stabilizes. During this time, normal operation of the

microcontroller is inhibited. After the state of the POR reaches a certain voltage value, the reset

delay time tPOR is invoked to provide an extra delay time after which the microcontroller can begin

normal operation.

VDD

Power-on

Reset

SST Time-out

tRSTD

Power-On Reset Timing Chart

● Low Voltage Reset – LVR

The microcontroller contains a low voltage reset circuit in order to monitor the supply voltage of

the device. The LVR function is always enabled with a specific LVR voltage VLVR. If the supply

voltage of the device drops to within a range of 0.9V~VLVR such as might occur when changing

the battery, the LVR will automatically reset the device internally and the LVRF bit in the CTRL3

range between 0.9V~VLVR must exist for a time greater than that specified by tLVR in the A.C.

characteristics. If the low supply voltage state does not exceed this value, the LVR will ignore the

low supply voltage and will not perform a reset function. The actual VLVR value can be selected

by the LVS bits in the LVRC register. If the LVS7~LVS0 bits are changed to some certain values

by the environmental noise, the LVR will reset the device after 2~3 LIRC clock cycles. When

this happens, the LRF bit in the CTRL3 register will be set to 1. After power on the register will

HT46R0664 Datasheet(PDF) 36 Page - Holtek Semiconductor Inc

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