SN74HC595: Logic forum

zac zhang

Part Number: SN74HC595
Other Parts Discussed in Thread: SN74HCS595, SN74LVC3G34, SN74LVC3G17

Tool/software:

I am using SN74HC595DR in my design as a shift register, sometimes I can't get it, so I want to use SN74HC595DRG4 instead, but I find that it doesn't work and the signal doesn't output correctly, I don't see any functional difference between these two.

SN74HC595DR has normal waveform, the shift cycles is 8 cycles, the output is normal:

I replace U13 and U16 with SN74HC595DRG4, the output of U13 is zero(QA-QH), and the QH_(PIN9) has abnormal output, the shift cycles is 2, which is not normal either.

I suspect there is a problem with the chip because the DRG4 chip I purchased does not have a logo on its surface, but I bought it from Mouser and I have asked this question on a forum before. The answer I received was that this is also normal. Later, I discovered that SN74HC595DR also has this type of chip without a logo

Can you give me guidance？Is the driving waveform of the chip correct? Why can't it output normally after replacing with DRG4？

11 months ago

0 Clemens Ladisch 11 months ago

Guru 316780 points

For newer chips, the letters "TI" are the correct logo.

I suspect that your CS_CLK waveform has ringing at the edges. Please show two waveforms, zoomed in so that it is possible to see the slope of a rising and a falling edge.

0 zac zhang 11 months ago in reply to Clemens Ladisch

Prodigy 95 points

Here are the rising and falling edge, does that have an impact?

0 Clemens Ladisch 11 months ago in reply to zac zhang

Guru 316780 points

That looks like multiple clock pulses for a device that is fast enough.

See p. 15…17 of this application note for how to properly terminate signal lines; you probably want source termination.

0 zac zhang 11 months ago in reply to Clemens Ladisch

Prodigy 95 points

What confuses me is why SN74HC595DR can be used, but SN74HC595DRG4 cannot be used if their parameters are the same

0 Clemens Ladisch 11 months ago in reply to zac zhang

Guru 316780 points

The newer devices are slightly faster, so the rising edges on the clock lines are actually detected as rising edges.

0 zac zhang 11 months ago in reply to Clemens Ladisch

Prodigy 95 points

That sounds reasonable. I'll try to eliminate the ringing first and see if it will return to normal, thanks.

0 zac zhang 11 months ago in reply to Clemens Ladisch

Prodigy 95 points

Hi Clemens, I tried to solve the ringing problem, but later I found that it was caused by the oscilloscope probe. After I replaced the oscilloscope probe and connected it to the pin as soon as possible, the ring was reduced to less than 1V, which should not affect the normal clk signal, but the output of the 595D chip is still abnormal, do you have any other suggestions? thanks.

0 Clemens Ladisch 11 months ago in reply to zac zhang

Guru 316780 points

You are observing that the '595 is shifting faster than expected. This implies that the '595 sees more clock edges than you are sending.

Please try sending a pattern like 10101010. If the output toggles multiple times, then you still have ringing.

0 zac zhang 11 months ago in reply to Clemens Ladisch

Prodigy 95 points

Hi Clemens, the clock frequency I am using is approximately 100KHz, and this device supports a maximum of 25MHz. If the clk speed is too slow, may it cause poor device performance?

0 Clemens Ladisch 11 months ago in reply to zac zhang

Guru 316780 points

The frequency has no lower limit. What matters is the speed of the signal edges. What is the slope of the edges now?

0 zac zhang 11 months ago in reply to Clemens Ladisch

Prodigy 95 points

Hi Clemens, I have optimized the CLK signal, and now there is no ringing, but there are still problems with the output. QA to QH are all 0, and the shift period of the QH_ is also wrong. Do you have any other suggestions?

0 Clemens Ladisch 11 months ago in reply to zac zhang

Guru 316780 points

QA…QH change when triggered with RCLK.

The last two input bits are 0, so when each clock pulse is read as four pulses, you get all zeros.

Might this chip be damaged? Does this still happen when you replace it?

What did you do to optimize the CLK signal?

0 zac zhang 11 months ago in reply to Clemens Ladisch

Prodigy 95 points

This is a common issue for SN74HC595DRG4, which own a "TI" letter logo. The 8 pieces I purchased cannot be used, and the waveform is similar. I purchased them from an official designated supplier, so the quality should not be a problem. Regarding the elimination of ringing, I connected a 33R resistor to the clk signal line.

0 zac zhang 11 months ago in reply to zac zhang

Prodigy 95 points

This is my all schematic, the input signal is reversed and isolated by optocoupler, the data chain is DAC+595D+595D, there are 25bits data every period, DAC retains 16 bits, do you currently prefer to think that this is a timing issue or a signal quality issue?

0 Clemens Ladisch 11 months ago in reply to zac zhang

Guru 316780 points

The rising edge still has ringing. And it happens near VCC/2, where it could leak to multiple clock edges.

I do not know if this is a measurement artifact of the probe, or actually happens at the pin. But your problem imples that it's the latter.

How long is the trace from the DAC to the 595? Where exactly did you place the 33 Ω?

Anyway, try using the SN74HCS595 instead; it has much larger hysteresis at its inputs.

0 zac zhang 11 months ago in reply to Clemens Ladisch

Prodigy 95 points

Hi Clemens, this is the layout and the location of the 33R.I am trying to buy and test S595 chip.

0 Clemens Ladisch 11 months ago in reply to zac zhang

Guru 316780 points

For source termination, the resistor should be as near as possible at the driver's pin.

0 zac zhang 11 months ago in reply to Clemens Ladisch

Prodigy 95 points

Hi Clemens, I used sn74HCS595DR for testing, unfortunately the output is still incorrect, similar to the phenomenon I encountered before. I am looking for the reason. Do you have any other suggestions or ideas

0 Albert Xu1 11 months ago in reply to zac zhang

TI__Mastermind 37975 points

Hi Zac,

As Clemens mentioned, do you still see this ringing on the rising edge?

0 zac zhang 10 months ago in reply to Albert Xu1

Prodigy 95 points

Hi Albert, it's still ringing on the rising edge. s595d does not seem to suppress oscillations.

0 zac zhang 10 months ago in reply to Albert Xu1

Prodigy 95 points

Hi Albert&Clemens, I have found the root cause, due to the long rise time of the input signal of SN74LVC3G34, uncertain signal appeared in the output, causing clock oscillation,do you have a recommended chip that can replace SN74LVC3G34 and accept long rise time input without causing output uncertainty? Thanks.

0 Clemens Ladisch 10 months ago in reply to zac zhang

Guru 316780 points

SN74LVC3G17

0 zac zhang 10 months ago in reply to Clemens Ladisch

Prodigy 95 points

Hi Clemens, thanks, I will try SN74LVC3G17.

By the way, since the ring's frequency is larger than 100MHz, does it mean that the SN74HC595DR can operate at frequencies larger than 100MHz?

0 Clemens Ladisch 10 months ago in reply to zac zhang

Guru 316780 points

Apparently, the '595 happens to be able to toggle its internal clock fast enough in this case. But it is not guaranteed, and I doubt that you can generate a reliable input signal at that speed.