shift register with or without latch

[lizxay]

Hi guys, i want to know the difference of a shift register with or without latch,i want to control the lightning of led,i've done experiment and the results are same. i dono if it will effect its efficiency,can u guys please help me on this? thank u and hope to hear from u soon =)

 

[Sceadwian]

I tend to prefer latching registers, but they require an extra control line. Usually there's four lines, clock, data, and two enable lines. When one enable line is on and you clock data in the shift register will fill with data, but nothing will change on the outputs of the shift register until you trigger the second enable line, at which point all the data on the internal register is pushed out onto the output latch. It's useful for large chains of LED where you don't want the possibility of flicker, as you can shift in a lot of data to say a dozen shift registers at once and then throw a master switch which updates them all.

 

[Martel]

The opposite is also true: Latched parallel to serial SR used for data acquisition for a microcontroller.

Suppose you have to monitor a large number of binary conditions, such as in a robotic arm (Base rotation ADC, shoulder position ADC, elbow position ADC, X and Y wrist position ADC's, wrist rotation ADC, Grabber position ADC, different strain gage ADC's (for force measurment)), you can easily end up with a hundred binary bits. You don't want to use one microcontroller pin for each of those bits !

A latched parallel in-serial out shift register is perfect for that. You use the Latch line to latch (acquire) all the data at say, 20 or 100 times per second and then, using the Clock line, you transfer all those data bits, once at a time to the microcontroller via the Data line. An interesting side of Shift Registers is that they can be daisy-chained one to another by using a Data In pin. All of the SR's Data Out pin is attached to the Data In of the next SR in the chain and the Data Out of the last one is sent to the microcontroller. In my robotic arm example, the Wrist-grabber end effector generates a lot of data, but by using a SR locally, you only need five lines for data acquisition: Data out, Clock in, Latch in, Vdd and Ground. At the elbow, you'll have six lines (including Serial Data in from the End effector) and so on.

Hope that will help !