In 8051, by default:
all interrupts are disabled
8051 Interrupts: Understanding the Default State
The 8051 microcontroller is a popular 8-bit embedded system chip that has various built-in features, including interrupts. Interrupts are special signals that can temporarily stop the main program execution to handle an urgent event or task. Once the event is handled, the microcontroller returns to its original program flow.
Default Interrupt State in 8051
When an 8051 microcontroller is powered on or reset, its internal registers are set to their default values. This includes the Interrupt Enable (IE) register, which is crucial for controlling whether specific interrupts are active or not. The IE register has individual bits that allow you to enable or disable different interrupt sources.
By default, upon a system reset, all bits in the IE register are cleared to logic '0'. This means that all interrupts, regardless of their source (like timer interrupts, external interrupts, or serial port interrupts), are initially disabled. To use any interrupt, you must explicitly enable it by setting the corresponding bit in the IE register to '1'.
Interrupt Enable (IE) Register Explanation
The IE register is an 8-bit Special Function Register (SFR) at address 0xA8H. Each bit in this register controls the enable/disable status of a specific interrupt source. The most significant bit (MSB), EA (Enable All), acts as a master switch for all interrupts. If EA is '0', no interrupt will be serviced, even if individual interrupt bits are set.
| Bit | Symbol | Description |
|---|---|---|
| IE.7 | EA | Enable All (Global Interrupt Enable/Disable) |
| IE.6 | (Reserved) | |
| IE.5 | ET2 | Enable Timer 2 Interrupt (8052 only) |
| IE.4 | ES | Enable Serial Port Interrupt |
| IE.3 | ET1 | Enable Timer 1 Interrupt |
| IE.2 | EX1 | Enable External Interrupt 1 |
| IE.1 | ET0 | Enable Timer 0 Interrupt |
| IE.0 | EX0 | Enable External Interrupt 0 |
Upon reset, the entire IE register is cleared to 00H. This means that EA (IE.7) is '0', and all individual interrupt enable bits (ET0, EX0, ET1, EX1, ES, ET2) are also '0'. Therefore, no interrupt can occur until the programmer explicitly enables them by setting the relevant bits in the IE register, including setting EA to '1'.
Analyzing the Options
- only timer/counter0 interrupt is disabled: This is incorrect. While timer/counter0 interrupt is indeed disabled, it's not the *only* one. All other interrupts are also disabled by default.
- all interrupts are disabled: This statement is accurate. Upon reset, the IE register is cleared, setting the global interrupt enable bit (EA) and all individual interrupt enable bits to '0'. This ensures that no interrupt can interrupt the CPU's operation until explicitly enabled by software.
- only System RESET interrupt is disabled: This is incorrect. The System RESET is a non-maskable event that initializes the microcontroller, it's not an interrupt that can be enabled or disabled in the same way as other maskable interrupts. The question refers to the maskable interrupts controlled by the IE register.
- only timer/counter1 interrupt is disabled: This is also incorrect for the same reason as option 1. All interrupts, not just timer/counter1, are disabled by default.
Therefore, the statement that all interrupts are disabled by default in the 8051 microcontroller is the correct one.
Important Questions from Microprocessors
An 8086 address bus is a/an _____ -bit bus
Match the columns.
Pin
Description
a. D 0-D 7
1. Reset Input
b. RESET
2. Data Lines
c. A 0,A 1
3. Internal Address
An 8 bit microcontroller has an external RAM with the memory map from 8000 H to 9FFF H. How many number of bytes can this RAM store?
In 8085 microprocessor, initially the number decimal 8 is stored. If instruction RAL is executed twice on this number, the final number stored will be
Which of the following 8085 microprocessor instructions occupies 3 bytes in memory?
