Forums

USB pin connections

Started by Unknown November 27, 2007
I'm new at working with USB and need some help with a design issue.

My uC with USB functionality (Cypress PSoC  CY8C24994) needs to
operate on 3.3V but I am aware that the USB Vcc is traditionally 5V.
PSoC Designer allows me to start up the uC in 3V USB operation mode,
which effectively "disables voltage regulator and pass-through Vcc for
pull-up", but does not indicate whether or not to physically
disconnect the Vcc pin of the USB port from the 3.3V power line that
the uC runs on. (Original data sheet says to connect the two powers,
which I assume was for standard 5V mode).

Is there any reason for me to leave the USB 5V Vcc connected to the
3.3V power line of the uC? I also am wondering if anyone knows from
experience whether or not the D- and D+ pins for USB will be able to
operate correctly if I operate in 3.3V mode.

Thanks,
Brian
On Nov 27, 8:38 am, bgon...@gmail.com wrote:

> I'm new at working with USB and need some help with a design issue. > > My uC with USB functionality (Cypress PSoC CY8C24994) needs to > operate on 3.3V but I am aware that the USB Vcc is traditionally 5V. > PSoC Designer allows me to start up the uC in 3V USB operation mode, > which effectively "disables voltage regulator and pass-through Vcc for > pull-up", but does not indicate whether or not to physically > disconnect the Vcc pin of the USB port from the 3.3V power line that > the uC runs on. (Original data sheet says to connect the two powers, > which I assume was for standard 5V mode). > > Is there any reason for me to leave the USB 5V Vcc connected to the > 3.3V power line of the uC? I also am wondering if anyone knows from > experience whether or not the D- and D+ pins for USB will be able to > operate correctly if I operate in 3.3V mode.
I wouldn't connect the USB Vcc to the controller, if it expects 3.3V. The D-/D+ signalling levels are compatible with 3.3V operation. Minimum output voltage for a 'high' signal is specified as Voh (min) = 2.8V. See sections 7.1.7.1 and 7.3.2 of the USB 2.0 spec.
On Nov 26, 11:38 pm, bgon...@gmail.com wrote:
> I'm new at working with USB and need some help with a design issue. > > My uC with USB functionality (Cypress PSoC CY8C24994) needs to > operate on 3.3V but I am aware that the USB Vcc is traditionally 5V. > PSoC Designer allows me to start up the uC in 3V USB operation mode, > which effectively "disables voltage regulator and pass-through Vcc for > pull-up", but does not indicate whether or not to physically > disconnect the Vcc pin of the USB port from the 3.3V power line that > the uC runs on. (Original data sheet says to connect the two powers, > which I assume was for standard 5V mode). > > Is there any reason for me to leave the USB 5V Vcc connected to the > 3.3V power line of the uC? I also am wondering if anyone knows from > experience whether or not the D- and D+ pins for USB will be able to > operate correctly if I operate in 3.3V mode. > > Thanks, > Brian
Take a look at page 22 of the data sheet. The processor is pretty tolerant of power supply ranges. If it were me, I'd look at the schematics of the evaluation boards, and "do what they do". G.