Peripheral Interfacing is considered to be a main part of Microprocessor, as it is the only way to interact with the external world. The interfacing happens with the ports of the Microprocessor.
The main IC's which are to be interfaced with 8085 are:
1. 8255 PPI
2. 8259 PIC
3. 8251 USART
4. 8279 Key board display controller
5. 8253 Timer/ Counter
6. A/D and D/A converter interfacing.
PROGRAMMABLE PERIPHERAL INTERFACE - INTEL 8255
Pins, Signals and internal block diagram of 8255:
It has 40 pins and requires a single +5V supply.
The INTEL 8255 is a device used to parallel data transfer between processor and slow peripheral devices like ADC, DAC, keyboard, 7-segment display, LCD, etc.
The 8255 has three ports: Port-A, Port-B and Port-C.
Port-A can be programmed to work in any one of the three operating modes mode-0, mode-1 and mode-2 as input or output port.
Port-B can be programmed to work either in mode-0 or mode-1 as input or output port.
Port-C (8-pins) has different assignments depending on the mode of port-A and port-B.
If port-A and B are programmed in mode-0, then the port-C can perform any one of the following functions.
As 8-bit parallel port in mode-0 for input or output.
As two numbers of 4-bit parallel ports in mode-0 for input or output.
The individual pins of port-C can be set or reset for various control applications.
If port-A is programmed in mode- 1/mode-2 and port-B is programmed in mode-1 then some of the pins of port-C are used for handshake signals and the remaining pins can be used as input/ output lines or individually set/reset for control applications.
The read/write control logic requires six control signals. These signals are given below.
1. RD (low): This control signal enables the read operation. When this signal is low, the microprocessor reads data from a selected I/O port of the 8255A.
2. WR (low): This control signal enables the write operation. When this signal goes low, the microprocessor writes into a selected I/O port or the control register.
3. RESET: This is an active high signal. It clears the control register and set all ports in the input mode.
4. CS (low), A0 and A1: These are device select signals. They are,
Interfacing of 8255 with 8085 processor:
A simple schematic for interfacing the 8255 with 8085 processor is shown in fig.
PIN DESCRIPTION - INTEL 8255
Block diagram of 8255:
The internal block diagram of 8255 is shown in fig:
The 8255 can be either memory mapped or I/O mapped in the system. In the schematic shown in above is I/O mapped in the system.
Using a 3-to-8 decoder generates the chip select signals for I/O mapped devices.
The address lines A4, A5 and A6 are decoded to generate eight chip select signals (IOCS-0 to IOCS-7) and in this, the chip select IOCS- 1 is used to select 8255.
The address line A7 and the control signal IO/M (low) are used as enable for the decoder.
The address line A0 of 8085 is connected to A0 of 8255 and A1 of 8085 is connected to A1 of 8255 to provide the internal addresses.
The data lines D0-D7 are connected to D0-D7 of the processor to achieve parallel data transfer.
The I/O addresses allotted to the internal devices of 8255 are listed in table.