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Demultiplexing of address and data bus in 8085

Last updated on August 4th, 2022 at 08:29 pm

In this post, we will see how Demultiplexing of multiplexed address and data bus AD7-AD0 in 8085 is done. The 8085 uses a multiplexed data bus and address bus. This means the Lower address bus (A7 – A0) and the data bus (D7 – D0) are available on the same pins AD7 – AD0.

Multiplexing is done to reduce the number of pins of 8085. However, external memory or I/O devices need the separate 16-bit address A15 – A0 for decoding and selecting a device and separate D7 – D0 for data transfer.

So, the multiplexed address-data bus must be demultiplexed so that the memory and I/O devices will have a separate address bus A15 – A0 and data bus D7 – D0. The demultiplexing can be done with the help of the ALE signal (Address Latch Enable) given out by the processor for this purpose.

Demultiplexing of address and data bus in 8085 with ALE signal

Pin 30 of 8085 is the ALE pin which stands for ‘Address Latch Enable’. ALE signal is used to demultiplex the lower order address bus (AD0 – AD7).

Pins 12 to 19 of 8085 are AD0 – AD7 which represents the multiplexed address-data bus.

Multiplexing is done to reduce the number of pins of 8085. The lower byte of address (A0 – A7) is available from AD0 – AD7 (pins 12 to 19) during the T1 of the machine cycle. But the lower byte of address (A0 – A7), along with the upper byte

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A8 – A15 (pins 21 to 28) must be available during T2 and the rest of the machine cycle to access memory location or I/O ports.

Now ALE signal goes high at the beginning of T1 of each machine cycle and goes low at the end of T1 and remains low during the rest of the machine cycle.

This high to low transition of ALE signal at the end of T1 is used to latch the lower order address byte (A0 – A7) by the latch IC 74LS373 so that the lower byte A0 – A7 is continued to be available till the end of the machine cycle.

The situation is explained in the following figure:

figure 1:

The Lower byte of address latching is achieved by the H to L transition of the ALE signal, which occurs at the end of T1 of each machine cycle.

Why the lower-byte address bus (A0 – A7) and data bus (D0 – D7) are multiplexed?

This is done to reduce the number of pins of 8085, which otherwise would have been a 48 pin chip. But because of multiplexing, external hardware is required to de-multiplex the lower order multiplexed address-data bus.

Author of this post

This post is co-authored by Professor Saraswati Saha, who is an assistant professor at RCCIIT, a renowned degree engineering college in India. Professor Saha teaches subjects related to digital electronics & microprocessors.

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