The 74HC245 is a 8-bit transceiver, I wanna use it to convert 3.3V to 5V signal and vise versa. I can use the DIR pin to toggle the direction of conversion according to this table below


But what confuses me is that, people state that the output voltage is equal to the voltage supplied to the Vcc pin. So when I convert 3.3V to 5V I can provide +5V to Vcc pin, but when I want to convert 5V to 3.3V should I also change the supply voltage to Vcc pin (yes, I sound stupid to myself)? Or can i just toggle the state of the DIR pin.

 

hi,
Vout is set by the Vcc supply.
DIR is only the direction.
E

 

You can't use it as a bidirectional level converter. That is not what it is designed for.
Irrespective of the direction, the output will be just about the Vcc level.

 

You can't use it as a bidirectional level converter. That is not what it is designed for.
Irrespective of the direction, the output will be just about the Vcc level.

"Cannot be used as a bi-directional converter"??

Then what is the point of having DIR pin. Sorry my knowledge about this IC comes from the link below

https://components101.com/ics/74hc245-pinout-specs-equivalent-datasheet

I am not able to make out much from the Datasheet. But I have read somewhere that this IC can be used for I2C communication between a 5V and 3.3V device so it has to be bi-directional right? What am I missing here?

 

it is a bidirectional transceiver but... it is NOT a bi-directional LEVEL converter. to convert level, it would need two separate supply voltages.

 

it is a bidirectional transceiver but... it is NOT a bi-directional LEVEL converter. to convert level, it would need two separate supply voltages.

Thanks, Now it makes sense

 

be also aware of which 5v side or the 3.3v side changes the direction, also consider if any of the inputs are logically floating when in the direction convertsion

Check the 74LVC8T245 level converter

also check if you need level bus hold on the converter , if so look at 74LVCH8T245

 

"Cannot be used as a bi-directional converter"??

Then what is the point of having DIR pin. Sorry my knowledge about this IC comes from the link below

https://components101.com/ics/74hc245-pinout-specs-equivalent-datasheet

I am not able to make out much from the Datasheet. But I have read somewhere that this IC can be used for I2C communication between a 5V and 3.3V device so it has to be bi-directional right? What am I missing here?

There was something wrong in the description of the linked page:

* Maximum Input Voltage: 4.2V

How come a Vin less Vcc of the IC?
Normally it will shows as VI < -0.5 V or VI > VCC + 0.5 V[on page 4]

The 74HC245 can do the converting job for 3.3V to 5V, because its voltage level caused it, when the Vcc is 5V then the Vin minimum high voltage = 2.67V(typical, see ViH on page 5), so it can be accept 3.3V as a high level, but it doesn't meant it is a voltage level converter, and it can't do the converting job as 5V to 3.3V.

 

be also aware of which 5v side or the 3.3v side changes the direction, also consider if any of the inputs are logically floating when in the direction convertsion

Check the 74LVC8T245 level converter

also check if you need level bus hold on the converter , if so look at 74LVCH8T245

The 74HC245 can't be a bidirectional level converter, because the bidirectional buffer all supply by the same Vcc, and it is only provided one Vcc, please check the pin 20 of page 3 on 74HC245 datasheet, but the 74LVCH8T245 provided two Vcc that they were included Vcc(A) and Vcc(B), please check the 4. Functional diagram of page 2 and page 3 on 74LVCH8T245 datasheet.