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Quickstart schematic questions — Parallax Forums

Quickstart schematic questions

dr hydradr hydra Posts: 212
edited 2013-03-21 14:39 in Propeller 1
I am looking at the schematic for the quickstart board and have a few questions

1. the 5v VBUS...is that 5v regulated? Can you take the VBUS and use that as a 5v power supply for a 5v device (microcontroller)? And is that true for all VBUS (from computers, wall chargers, etc)? It looks like the FT232RL gets a direct feed (VCC) from the USB device...does the FT232 a have regulator in the chip for the 5v?

2. Why does the quickstart board use a 74lvc126 buffer...Why not just feed the 3v3out from the FT232RL to the VCCIO then all the input/output from the FT232 would be at 3.3v...therefore, elimating the 74LVC126 chip

Thanks

Comments

  • tonyp12tonyp12 Posts: 1,951
    edited 2013-03-21 06:56
    The 5V is regulated if YOU use a regulated supply, 5v from PC's USB cable or charger is regulated.

    The buffer is because weird reset problems, parasitic power would flow back in to the non-powered ftdi chip and wake it up and cause a reset.
  • dr hydradr hydra Posts: 212
    edited 2013-03-21 07:10
    tony

    thank you for the reply...so (most likely) my usb phone charger that plugs into the wall is a regulated 5v supply? The reason I ask is because...I remember (in the past) hearing that wall plug power supplies (like the 9VDC 300ma power supply sold by parallax) is unregulated and can cause a spike in power which can damage your board.
  • blittledblittled Posts: 681
    edited 2013-03-21 10:39
    The USB phone charger is 5V regulated otherwise it would have power spikes damaging the phone. As for the 3.3V from the FT32, I believe it can only handle 30mA of current while the 5V can handle 500mA so using the 74LVC126 permits a higher current source. Wall power plugs can damage sensitive circuits with power spikes. I used a 9V one once and blew out a Serial LCD that could handle 9V unregulated.
  • dr hydradr hydra Posts: 212
    edited 2013-03-21 11:53
    great info...I was not sure if the USB phone charger was 5V regulated or if the phones had 5V protection circuits.
  • dr hydradr hydra Posts: 212
    edited 2013-03-21 13:48
    looking at the schematic again...what is the USB-PWR-EN port...is that just a 5v power supply when the board is connect to the usb port (PC or charger)?
  • tonyp12tonyp12 Posts: 1,951
    edited 2013-03-21 14:09
    The p-channel DMP2035U has a 10k pull-up resistor to 5+Vbus on it's gate.
    By connecting the USB-PWR-EN to gnd, you will override this 5V signal
    So by grounding it, you enable Vbus to be available on Vin.

    P-Mosfet
    http://www.diodes.com/datasheets/ds31830.pdf
    VbusEnable.gif
    VbusEnable.gif
    543 x 402 - 30K
  • dr hydradr hydra Posts: 212
    edited 2013-03-21 14:15
    cool...when the board is plugged into the USB charger or pc...is there a 5v power supply port available...VIN?
  • tonyp12tonyp12 Posts: 1,951
    edited 2013-03-21 14:39
    Yes, and there is also two 10k res going to CBUS3 (default PWREN#),
    But a grounded USB-PWR-EN will override any signal comming from FTDI chip.
    Quote:
    Output is low after the device has been configured by USB, then high during USB suspend mode.
    This output can be used to control power to external logic P-Channel logic level MOSFET switch.
    Enable the interface pull-down option when using the PWREN# in this way.
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