Why 8255

Check the arithmetic instructions for Also, check the logical instructions for You will notice that there are no direct instructions to perform arithmetic or logical operations with data stored at a port address. Hence, for any arithmetic or logical operation to be performed on data from peripherals, we first have to use the IN command, get the data into the ALU, then move it to a register and then perform the operation.

So to answer your question in short. You have to jump through a few hoops to do it. I hope that answers your question. Good question! This site uses Akismet to reduce spam. Learn how your comment data is processed.

Obaidur Rahman. Digital Electronics Course A free course on digital electronics and digital logic design for engineers. Leave a Reply Cancel reply. Control words and status information are also transferred through the data bus buffer. The data lines are connected to BDB of p. The function of this block is to control the internal operation of the device and to control the transfer of data and control or status words.

It accepts inputs from the CPU address and control buses and in turn issues command to both the control groups. A low on this input enables the to send the data or status information to the CPU on the data bus. They are normally connected to the least significant bits of the address bus A 0 and A 1.

Following Table gives the basic operation,. This is done to prevent destruction of circuitry connected to port lines. If port lines are initialized as output after a power up or reset, the port might try to output into the output of a device connected to same inputs might destroy one or both of them. The A contains three 8-bit ports A, B and C. All can be configured in a variety of functional characteristic by the system software. PORT B:.

PORT C:. This port can be divided into two 4-bit ports under the mode control. More specifically we can say that the data transfer operation gets enabled by an active low signal at this pin.

RD — It is the signal used for read operation. A low signal at this pin shows that CPU is performing read operation at the ports or status word. Or we can say that is providing data or information to the CPU through data buffer. WR — It shows write operation. A low signal at this pin allows the CPU to perform write operation over the ports or control register of using the data bus buffer.

A 0 and A 1 : These are basically used to select the desired port among all the ports of the and it do so by forming conjunction with RD and WR. It forms connection with the LSB of the address bus. Reset : It is an active high signal that shows the resetting of the PPI. A high signal at this pin clears the control registers and the ports are set in the input mode.

Initializing the ports to input mode is done to prevent circuit breakdown. As in case of reset condition, if the ports are initialized to output mode then there exist chances of destruction of along with the processor. As we can see that pin number 27 to 34 is allotted to data bus.

These are tri-state data bus lines of bidirectional nature that connects with the processor. These are used to allows flow of data and control or status word between the and the processor. The external voltage required to drive the circuit is provided at pin number 26 i.