Before reading this article, you should have a basic grasp of survivalcraft electrics and understand how the Memory Bank works and how to program it. Also note that in these circuits the clock input of the bank is NOT used to write to the banks, so the ‘funniness’ of writing is not necessary.
It is always important to remember that the timing of the memory bank’s CLOCK input and its relationship to other signals could be critical. It is very wise to have an understanding of synchronous systems, such as most computers, when designing with latches and registers.
Introduction
A latch is a single memory location that will store one value for later recall. They are often used with an input that changes over time and triggered on a specific event when you want to remember what the input was at that moment in time. They are typically used in electric circuits to keep an input steady while it is read, or to hold an output until it is read by another circuit, or by the user.
A register is simply a latch that can be read back by the same circuit that set it. When latches are used in computer circuits they are often simply called registers.
Basic Latch
The basic latch simply locks in the value of the input whenever the control signal goes high (a digital '1'). The bank is programmed linearly in the '0' row so the output value is the same as the low address input value.
To show that the ‘Latch/’ control activates on the transition, the input connection is shown with a triangle. The “/” at the end of the name also shows that it activates on the rising edge of the signal. The other inputs, which are state dependent, use a square. --- To program a linear response, put “0123456789ABCDEF” in the first row. If you need to use the High Address input for the Data Input, put that string down the first column, instead.
Buffered Latch
The buffered latch adds the ability to disable the output and only allow it through when the output enable control is active (digital '1'). This works because Survivalcraft electric gates use an implicit OR in each connection. That means any number of outputs may be wired together and each one is effectively OR'd with all the others.
The buffered latch is able to have the output and input connected together. The value of the input only matters when the latch control goes to '1'. During that time, the output enable must be off (digital'0'). This lets the latch be connected to a data bus and only be activated when needed.
This has an internal data output for any circuitry that needs access to the data regardless of the enable control.
Decoded Latch
This circuit replaces the AND with a Memory Bank and is more typical of a register used in a computer system. The bank is programmed to operate the same as the AND but only when the proper value (address) is present on the enable input. This address can be either one or both of the address inputs depending on the circumstances.
For a full bidirectional register, the latch control must also be address decoded and set to the same address. Simply put the linear response ‘code’ in the row that corresponds to the address you wish to use to read the register. Remember that the Output Enable Address input must decode the ENTIRE address space. I.e., in a 8-bit address, you cannot simply use the lower 4 bits alone – the upper address bits MUST also be included. (See the decoding page.)
Bus Based Register
In a bus based computer system, the input and output of the register usually needs to be accessed at the same memory location. The following circuit adds the final stage of memory decoding as an example.
This way there is no separate line for the output enable and the latch control. They both incorporate the system R/-W signal. This signal is typically locked to the clock in a synchronous system. The last stage of a typical address decoder is shown as well. This stage could be used to control the timing of the latch control, if the R/-W signal does not. This would be a part of the ‘bank select’ signal.
Note that you could save space in this circuit by using another memory bank to replace the NOT gate and the AND gate that it feeds into. If we assume the NOT goes into the High Address input and the decoded address into the Low Address Input, then “000000000000000F” is programmed into the first row.
Uses
- Latches are often used in video games to capture the value of some variable at the moment the player hits a button.
- They are used to hold a value for display while the circuit can continue with other functions.
- They may be used to stop and freeze a changing input, so it can be read without jitters.
- The inputs to and output from a computer's ALU are held in registers.
Notes
- You must always remember that the latch activates only AS the latch control goes to '1'. This timing may be critical and the other inputs must be stable at that moment.