Maximum Switching Speed Chained 74HC595

Question

I'm planning to use a set of daisy chained 74HC595s to control a set of 108 mosfets, 3 of them are geographically in the same area (couple of mm apart) and for the sake of modularity should each use their own register. That would mean I will use 36 74HC595s and chain them together. They are all on one board and the distance between the groups of 3 is around 2-5cm.

Questions:

1. How do I calculate the minimum time it takes to to write the whole set of serial instructions to 36 74HC595? I figured I need to add clock rise times and propagation times, but I'm not sure exactly how.
2. Is there anything else I need to add for this to work? Do I need additonal buffers or delay components? There's gonna be some noise made by high currents (through the mosfet).

Answer 1

It would appear that you intend to operate these 13 shift registers using software to bit-bang some GPIO pins of a microcontroller to operate the SER_IN, SER_CLK and REG_CLK of the 74HC595s. It would be unlikely that in this scenario that you would be able to generate the control signals fast enough to exceed the capabilities of the shift registers.

From a pure analytical standpoint, if you can build the circuit to ensure that the SER_CLK signal is essentially the same signal to all of the shift registers then the maximum shifting frequency will be based upon the same specifications of just the first shift register. If there is delay of the SER_CLK getting from shift register to shift register due to long signal traces then you would want to total up that overall delay from end to end and use that as a guide to understand the fastest positive edge to positive edge to next positive edge of the SER_CLK.

I had a recent project there I operated a series of 8 such shift registers driving a total of 64 LEDs in an 8x8 matrix. I was using a microprocessor running at 22.1184 mHz and writing the tightest code possible to bit-bang the control signals I never even came close to overclocking the shift registers.

You will have to use very careful design with handling the high current paths of your MOSFETs. If you get significant GND noise it could impair the signal integrity at the shift registers and cause them to not operate correctly. Do keep in mind that this type of noise and its ability to disrupt proper operation of the shift registers will have very little to do with how fast you operate the shift register control signals.

Answer 2

I daisychained 45 shift registers (TPIC6B595, high current sink version of 74HC595) with Arduino, system clock 16 MHz, using built-in SPI hardware to send data out with 8 MHz SPI clock. The shift register clock ran at 8 MHz. I was able to achieve 20 KHz update rate. SN74HC595 (TI brand) has a max clock of 25 MHz, so with a faster processor you should be able to go 3 times faster. With Arduino, it takes 17 clocks to output a byte. At 16 MHz, that's 62.5nS/clock, so you could figure it will take

36 shift registers * 17 clocks/shift register * 62.5nS/Clock = 38 microseconds

With a faster processor, you could push that down by ~ 1/3.

For layout comparison, I had 5 boards with 9 shift registers each, and each board had the shift register clock, data out line, and latch line buffered to break up the capacitance seen by the microcontroller.