Cascading

Cascading

The 8259A can be easily interconnected to get multiple interrupts. Fig. 9.8.1 shows how 8259A can be connected in the cascade mode. In cascade mode one 8259A is configured in Master mode and other should be configured in the Slave mode. In this figure, 8259A-1 is in the master mode and others are in slave mode. Each slave 8259A is identified by the number which is assigned as a part of its initialization. Since the 8085 has only one INTR input, only one of the 8259A INT pins is connected to the 8085 INTR pin. The 8259A connected directly to the 8085 INTR pin is referred to as the master. The INT pins from other 8259As are connected to the IR inputs of the master 8259A. These cascaded 8259As are referred to as slave. The  signal is connected to both master and slave 8259A.

The cascade pins CAS₀ to CAS₂ are connected from the master to the corresponding pins of the slave. For the master these pins function as outputs, and for the slave these pins function as inputs. The  signal is tied high for the master. However, it is grounded for the slave.

Each 8259A has its own addresses so that command words can be written to it and status bytes read from it.

Addresses for 8259As:

Master and Slave Operation :

When the slave receives an interrupt signal on one of its IR inputs, it checks mask condition and priority of the interrupt request. If the interrupt is unmasked and its priority is higher than any other interrupt level being serviced in the slave, then the slave will send an INT signal to the IR input of a master. If that IR input of the master is unmasked and if that input has higher priority than any other IR inputs currently being serviced, then the master will send an INT signal to the 8085 INTR input. If the INTR interrupt is enabled, the 8085 will go through its INTR interrupt procedure and sends three  pulses to both the master and the slave. In response to first interrupt acknowledge signal opcode for CALL instruction is put on the data bus and the master outputs a 3-bit slave identification number on the CAS₀-CAS₂ lines. Sending the 3-bit ID number enables the slave. When the slave receives the second   pulse from the 8085, the slave will send the low-order address byte of the ISR on the data bus. Finally, slave sends the high-order byte of the ISR on the data bus on receiving third   signal.

If an interrupt signal is applied directly to one of the IR inputs of the master, the master will send the opcode for CALL instruction to the 8085 when it receives the first   pulse from the 8085. It then sends low-order byte and high-order byte in successive interrupt acknowledge cycles (second and third).

Review Question

1. Draw and explain the interfacing of cascaded 8259s with 8085.