Pullup in HC05 with atmega

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Hi, i am using atmega128 with Bluetooth module HC05. I was having No trouble at all with the communication but reccently a new batch of bluetooth module arrived and with the same configuration when i replaced the new BT with the previous one, it doesn't recieve data properly. I found out the problem and placed a pullup resistor on my microcontroller RX and and Vcc and it sort of worked... My question is that does the BT module have a built in pullup resistor because previously it was working without Pullup resistor but the batch that recently arrive not a single one of them work without pullup resistor. and is there a way to check the presense of pullup resistor in bluetooth module HC05 .

Thanks 

Last Edited: Wed. May 8, 2019 - 04:47 AM
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The interface of HC05 is 3.3V level.
Such things can happen if the atmega128 side does not follow it.
If you are using at 5V, you have to insert a level conversion.

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My question is that does the BT module have a built in pullup resisto

Not likely...since HC05 contains a logic gate output (TX) it will will drive hi/lo like any typical logic gate output (excepting things like open collector or open drain style gates).

It's possible they might have some resistors to define the levels during HC05 power off or reset, but not likely--they want to sell as cheaply as possible.

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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Do not sell something that is not designed correctly.
It will be a very annoying product.

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avrcandies wrote:
like any typical logic gate output (excepting things like open collector or open drain style gates).

 

I connected only VCC and GND and checked if there was some internal connection to TX but there was no volatage on TX of HC05 , which confirms what you said , However i am curious how it worked flawlessly when i had no Pullup resistor for one BT module and didn't work for a whole different batch when they both have no sign of internal pullups.

Last Edited: Wed. May 8, 2019 - 08:00 AM
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kabasan wrote:

Do not sell something that is not designed correctly.
It will be a very annoying product.


Tell that to Microsoft.........

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Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB, RSLogix user

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Post a complete, and accurate, schematic diagram of your project.

 

JC