Question

Describe the function of the following pins of 8051: (i) PSEN (ii) RESET (iii) ALE (iv) EA.

Answer 1

Here's a simple explanation of the function of the following pins in the 8051 microcontroller:

1. PSEN (Program Store Enable):

   - Function: The PSEN pin is used to enable the external program memory. When the microcontroller is fetching an instruction from external memory (usually ROM or Flash), this pin is active. It is an output pin and goes low (0) during the fetch cycle, indicating that the external program memory is being accessed.
   - Key Point: This pin is active low.

2. RESET:

   - Function: The RESET pin is used to initialize the microcontroller. When a high signal is applied to this pin, the 8051 resets itself, and all the registers and memory are set to their initial values. It essentially restarts the microcontroller, making it ready to begin execution from address 0.
   - Key Point: This pin is active high. When a high voltage is applied, the microcontroller is reset.

3. ALE (Address Latch Enable):

   - Function: ALE is used to latch the low-order address byte (A0-A7) into an external latch. The 8051 has a 16-bit address bus, but it can only output 8 bits at a time. The ALE pin is used to latch the lower 8 bits of the address (A0-A7) during the fetch cycle to external memory. This allows the microcontroller to access addresses higher than 255.
   - Key Point: This pin is used to separate the low-order address during memory access. It is active low.

4. EA (External Access):

   - Function: The EA pin determines whether the microcontroller should fetch code from external program memory or internal program memory.
     - If EA is tied high, the microcontroller fetches instructions from the internal ROM.
     - If EA is tied low, the microcontroller fetches instructions from external program memory (like external ROM).
   - Key Point: This pin is used to select between internal or external program memory.

In summary:

1. PSEN: Enables external program memory.
   

1. RESET: Resets the microcontroller.
   

1. ALE: Latches the low-order address.
   

1. EA: Selects between internal and external program memory.