SN74HCT595: Operating Voltage confirmation

narendra singh1

Part Number: SN74HCT595

Tool/software:

Hi,

I am designing USB to Uart schematic based on FT4232H-56-REEL from FTDI. In datasheet, they have application circuit which uses part SN74HCT595D. As per datasheet, SN74HCT595 should operate starting from 4.5V minimum but the reference design uses 3.3V. I am wondering id=f the FTDI design is correct or not. I can understand since the part supports TTL level input, SPI communication can work at 3.3V logic level but how with 3.3V VCC circuit is working? They claim to selling development kit which uses the same part. section 6.4 4 Channel Transmit and Receiver LED Indication Example https://ftdichip.com/wp-content/uploads/2024/05/DS_FT4232H.pdf. In case their circuit is wrong, can we just supply 5V on VCC and connect other SPI signals on 3.3V logic itself?

Regards

Narendra Singh

2 months ago

+1 Clemens Ladisch 2 months ago

Guru 262920 points

HCT has TTL-compatible inputs that accept 3.3 V signals, but runs with a 5 V supply (and outputs 5 V).
HC runs at any voltage between 2 V and 6 V, but input and output voltages must match.

The datasheet wants you to use HC. Revision 2.09 changed from HCT to HC, but they forgot one reference.

Here is a search for '595 shift registers that run at 3.3 V; you can use any of them: ti.com/logic-voltage-translation/flip-flops-latches-registers/shift-registers/products.html#-1=595;false&0min=1.5;3

0 narendra singh1 2 months ago in reply to Clemens Ladisch

Prodigy 230 points

Hi Clemens,

Thanks for your clarification. It helped.

Is there any recommendations for R1,C1 values?

Regards

Narendra Singh

0 Clemens Ladisch 2 months ago in reply to narendra singh1

Guru 262920 points

The power-up state of the flip-flops is undefined.

But you do not need the power-up clear with the FT4232 because it will quickly set the LED states.

If you do want a low pulse on /SRCLR, then you have to size R1×C1 so that the /SRCLR rise time is much slower than the V_CC rise time.

Logic

Logic forum

SN74HCT595: Operating Voltage confirmation