SN74LVC8T245 Logic Input Issues

Sankar karuppannan

Other Parts Discussed in Thread: SN74LVC8T245

HI,

In my design, am using SN74LVC8T245. VCCA is connected to 3.3V, VCCB is connected to 5V and DIR, OE pin are connected GND.

We are converting 5V to 3.3V logic. The signals are connected to the connector. The issue is 5V logic input signal level is not going to 5V(I am getting 1.5V).

If disconnect 245, the input signal is reaching to 5V. I have checked the following things in the circuit.

1) Cin and Cout are matching.

2) no short circuit on the board.

Please help me out. How to solve this issues.

Thanks,

Sankar.K

over 14 years ago

0 Hattie over 14 years ago

TI__Genius 9085 points

Hello Sankar,

What is the load that the LVC8T245 is driving? What is the resistance of the load?

Do you have a schematic I could check? Or waveforms?

You have tried switching the device with a new device? Is the device the correct part? You can verify this by checking the topside marking given in the ordering information on the first page of the datasheet (NH245.)

0 Sankar karuppannan over 14 years ago in reply to Hattie

Prodigy 120 points

Hi,

Please find the schematic diagram below

Din signals are connected to the microcontroller and IN signals are from connector.

The connector is connected to other board. I have found, due to other device failure, 245 input pin got failed.

Thanks and Regards,

Sankar.K

0 Hattie over 14 years ago in reply to Sankar karuppannan

TI__Genius 9085 points

Have you resolved the issue? Was it due to the other device failure?

Please let me know if you have further questions.

0 vineetrvce over 10 years ago in reply to Hattie

Intellectual 565 points

Hi,

Just to add to my findings, since I am doing a prototype, I have wired my circuit as shown in this forum discussion: "SN74LVC8T245 Logic Input Issues"

I have redone my wiring to keep the wires from the level shifter to the controller (VCCB side) as small as possible. With the current settings, I am able to make the level shifters work with VCCB = 3.3V and 5V. But for some reason, the 1.8V is just not working.

Any clue, if this is caused by the wiring? Also all unused inputs are grounded. DIR and /OE pins are pulled up with 10k resistor to VCCA.

Regards,

Vineet.

0 vineetrvce over 10 years ago in reply to vineetrvce

Intellectual 565 points

Another observation, when I check the voltage levels on the VCCA = 3.3V (always) side, they are as follows:

when VCCB = 1.8V (not working)
-----------------------------------
SPI CLK = 0.19V
SPI CS = 2.35V
SPI MOSI = 0.23V
SPI MISO = 1.01V

but when VCCB = 3.3 V (working)
--------------------------------------------
SPI CLK = 0V
SPI CS = 3.3V
SPI MOSI = 0V
SPI MISO = 0V

Why is this happening? Ignore this post

0 Emrys Maier over 10 years ago in reply to vineetrvce

TI__Guru* 97456 points

Vineet,I am moving your post to the Voltage Level Translation forum for support.

Logic

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SN74LVC8T245 Logic Input Issues