USR register. Various bits in the UART¢s setup register, UCR2, determine if these flags can gener
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ate a UART interrupt signal. More details on these two registers and how they influence the opera
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tion of the UART interrupt can be found in the UART section of the handbook.
Programming Considerations
The interrupt request flags, TF, T0F, T1F, T2F, URF, EIF, EIF0 and EIF1, together with the interrupt
enable bits ETI, ET0I, ET1I, ET2I, EURI, EEI, EEI0 and EEI1, form the interrupt control registers
INTC, INTC0 and INTC1, which are located in the Data Memory. By disabling the interrupt enable
bits, a requested interrupt can be prevented from being serviced, however, once an interrupt re
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quest flag is set, it will remain in this condition in the INTC, INTC0 or INTC1 register until the corre
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sponding interrupt is serviced or until the request flag is cleared by a software instruction.
It is recommended that programs do not use the ²CALL subroutine² instruction within the interrupt
subroutine. Interrupts often occur in an unpredictable manner or need to be serviced immediately
in some applications. If only one stack is left and the interrupt is not well controlled, the original con
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trol sequence will be damaged once a ²CALL subroutine² is executed in the interrupt subroutine.
Reset and Initialization
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 condi
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tion 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. One of these registers is the Program Counter,
which will be reset to zero forcing the microcontroller to begin program execution from the lowest
Program Memory address.
In addition to the power-on reset, situations may arise where it is necessary to forcefully apply a re-
set condition when the microcontroller is running. One example of this is where after power has
been applied and the microcontroller is already running, the RES
line is forcefully pulled low. In
such case, known as a normal operation reset, some of the microcontroller registers remain un-
changed allowing the microcontroller to proceed with normal operation after the reset line is al-
lowed to return high. 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, similar to the RES
reset is implemented in situations where the power supply voltage falls below a certain threshold.
Reset
There are five ways in which a microcontroller reset can occur, through events occurring both inter
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nally 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
58
I/O Type MCU
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