Re: Can use 3.0V for USB?

Boki wrote:
Hi All,

I am modifying a product ( says Product A ) from USB connectoin to
Bluetooth.


OK....

It is very simple to do a RS232 replacement.

Not quite sure what you mean by this. There are RS-232 to USB
converters, for PCs that can be used to implement a serial port in a
PC. I don't see how this would be of much utility to your application
though. I don't see how you can say that converting between USB and
RS-232 is easy.

But the output originally is USB, that is a trouble. I need to use a
USB (host?) chip to ouput RS232 signal, and one more level shift for my
final Bluetooth UART input.

There are many differences between RS-232 and USB, besides the voltage
levels.
First, USB is a single master, multiple slave protocol that utilizes
differential transmission with packetized communication and different
types of transactions. RS-232 is a point to point, ground referenced
communication channel. Even if you solve the electrical differences
between the two, the communication protocols are wildly different and
so translating between the two would be very difficult.

My Bluetooth is 3V. The product A use USB as power, and I measure the
VDD and GND is 5V ( when original Prodcut A operating ).

Yes, the USB specification says that the power and ground supplied by
the USB bus is 5Vdc.
Does that mean I can only use 5V for product A ( so I need a PWM ? if I
use battery 3.0V)

If you need a different voltage you can use a regulator, with the type
depending on your power requirements. Keep in mind that there are
limits to the amount of power you are allowed to draw from a USB bus.

Does it possible to use 3V for Product A.

because I think every chip has its own operation range.

but my co-worker just tell me, 3V seems not enough, why say so ?
becaues it is not in USB range?

or he is wrong?

No Idea, since I don't even know what Product A is. If it is a
commercial product that is meant to be supplied by 5V it probably won't
work with a 3.3V supply.

Best regards,
Boki.

Good Luck!

.

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