I've very less knowledge in electronics, I've read that a latch is a simple circuit that stores 1 bit of data (or state).

A Latch is a circuit that has two stable states and can be used to store state information.

That means it is a noun or a simple thing.

But while reading microprocessors (8085), in Timing and Control, there's a signal called ALE (Adress Latch Enable). And I read this line -

The ALE is a pulse signal, used to latch the address from AD0-AD7.

Now in the line above, the term latch is used as a verb. I don't know what exactly mean by that line above. What is mean by latching something?

So here my question is what is a latch? And what is the meaning of the line above? And what is ALE?

  1. Latch can be used as a noun or as a verb. You got it right.

  2. ALE - Some IC's overlap Address bus and Data bus pins to minimize the number of pins needed. That means one pin can be used as an address pin or as a data pin depending on the current state of the bus. For the logic to work properly address value should be present for the whole read or write operation. To accomplish this the mentioned IC has latches (memory registers) inside. ALE signal stores the pins' state into internal address registers (latches) of this IC. After that the bus can be switched to deliver data on these same pins, that used to carry the address a moment ago.

  • 2
    \$\begingroup\$ Just a note in case you are googling around. This is called "address data multiplexing" or an "multiplexed address data bus" \$\endgroup\$ – caveman May 29 '17 at 14:14

In English, 'latch' is both a verb and a noun.

In digital electronics, the usual distinction is that a latch is a memory gate with a level control: control input at one level for 'input flows through to output', control at opposite level for 'output held'. This differs from a flip-flop, which is typically edge-triggered.

Your 8085 saves pins by multiplexing address bus lines A[7:0] and data bus D[7:0] onto its pins AD[7:0]. External memories and circuitry will need these as separate signals so the 8085 has an Address Latch Enable (ALE) pin to control a transparent latch that will demultiplex them. When ALE is high, AD[7:0] drive out address lines A[7:0] and after ALE goes low, AD[7:0] is used as a bidrectional data bus.

The Intel designers ensured that the ALE level and timing would work with a readily-available logic chip of its day: the 74LS373 (now 74HCT373).

The demultiplexing can also be done by 8 falling-edge triggered flip-flops. However, the address on AD[7:0] will not be passed to connected memory chips (e.g. SRAM, EPROM, parallel Flash) until ALE falls. With a transparent latch, the address on AD[7:0] will flow through to memory chips as soon as the 8085 outputs it. So using flip-flops means shorter memory cycles at the memories because they cannot start decoding the address and accessing the memory cell until later in the cycle.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.