Why do I need a Latch for external memory?
Submitted By: Craig Steiner FAQ Last Modified: 07/10/06
- The 8051 data bus is multiplexed with the lower 8 bits of the address bus. The 573 is used to seperate these two busses from each other to get a full 16 bit address bus and an 8 bit data bus again. In fact the 573 freezes the contents presented on it's inputs when the ALE pulse arrives, making the lower 8 bits of the address available on it's outputs. After that the 8051 switches to data bus mode to receive or send the data.
"Traditionally" the latch was a 373 device, which has a pretty horrible pinout when you are laying out an address/data bus on a PCB. The 573 has a nice "straight through" pinout, with all the ins lining up across the package with all the outs. The 573 means you can lay out a processor, latch, RAM and ROM on one side of a PCB with no links.
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