Sourcing current using Shift Registers

[soc]

Hello All,

I am working on a project and need to on/off 32 current source from outside a chip.
The easiest way would be to include 32 pads meanings 32 pins, which requires more space and increase cost.
So does any one knows a better way using Shift Registers, important is all the current sources have equal currents with +- 0.1% deviation.

Thanks..

 

[keith1200rs]

You have already answered your own question - a 32 bit shift register. Only two pins required - data plus clock. If you don't want them switching all over the place you will need a 32 bit latch as well and an extra 'load' pin.

Keith.

 

[soc]

What if I have 64 current sources and I want to control them externally. Using a 64 bit Shift Register won't be advisible because of the cost if I am right.
Can you suggest any detailed implementation example for reference.

Thanks

 

[keith1200rs]

I am not sure I understand the problem. A 64 bit shift register will not be large on an IC. I guess I don't really understand the problem. I have implemented a 48 bit shift register on an IC for clocking in the settings for an analog IC from a microcontroller. The only connections required were clock and data, although for my particular design we also included a reset input to clear the shift register and a 49th bit which was accessible externally on the chip. They were for testing and run-time checking. While the size of the shift register was not negligible because it was on 0.8um CMOS, it was only a small proportion of the overall IC size.

Keith.

 

[soc]

Do you have any literature where I can find how to implement a 64-bit Shift Register at the transistor level?

Thanks

 

[keith1200rs]

I used the digital cells from the fab, although I simulated them at the transistor level. What process are you using? They should have digital cells available. I will see if I can find a D type circuit at transistor level tomorrow.

Keith

 

[keith1200rs]

There are some circuits for D-types in the R Jacob Baker book, if you have it. I guess it will also be in many design books.

Other sources:

https://en.wikipedia.org/wiki/File:TSPC_FF_R.png

Or you can make them out of NAND gates:

**broken link removed**

Keith.

 

[soc]

Hello,
I implemented a 1-bit SR using Transmission gates, NAND and NOT.
If I need a 64-bit do I just need to connect 64 1-bit cells?
Also in my application I will be using to on/off current sources externally and I need a high efficency meaning deviation <1%.

Thanks for your feedback.

 

[keith1200rs]

Hello,
I implemented a 1-bit SR using Transmission gates, NAND and NOT.
If I need a 64-bit do I just need to connect 64 1-bit cells?

Yes. It would be easiest if you create a flip-flop subcircuit and symbol to save your circuit looking huge when you draw it out.

Keith.