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TMS320C5517: Excluding the USB peripheral from the design

Miroslav Bozic
Intellectual 420 points
Part Number: TMS320C5517
Other Parts Discussed in Thread: TMS320C5505

Hi everyone,

I have designed the hardware using mentioned TI DSP. The first release of the hardware has included USB peripheral and all components that are related was populated on board. Originally we have planned to use the USB for the communication, but after some discussion and first release, we figured out that there is no need for this peripheral at all. Currently, I am trying to optimize the cost and since the USB is not needed at all I would like to exclude all components and make it NI. This should be done without changing the PCB design and  I have some concerns that are listed below:

  1. Is it ok to leave USB_MXI, USB_MXO, and USB_VSSOSC unconnected? 
  2. Is it ok to leave USB_R1 and USB_VSSREF connected with the 10k resistor? There is no way to connect USB_VSSREF to the ground, it's not possible on the current design. 
  3. Is it ok to leave USB_DM and USB_DP unconnected?
  4. Is it ok to leave USB_VBUS unconnected?

Rest of the pins that are related to the USB peripheral will be connected as has been proposed in the DSP datasheet

USB_VDDA3P3 -> GND

USB_VDDA1P3 -> GND

USB_VDD1P3 -> GND

USB_VDDPLL -> GND

USB_VDDOSC -> GND

Thanks in advance, best regards.

  • 0
    TI__Mastermind 30110 points

    Hi Miroslav,

    Is it ok to leave USB_MXI, USB_MXO, and USB_VSSOSC unconnected?

    * USB_MXI should be connected directly to ground (VSS).

    * USB_MXO should be left unconnected.

    Is it ok to leave USB_R1 and USB_VSSREF connected with the 10k resistor? There is no way to connect USB_VSSREF to the ground, it's not possible on the current design.

    * It is recommended to connect USB_VSSREF directly to ground, but if it is not possible then keep it in mind for the next revision. I'll try to find if there are consequences for not grounding it (power consumption?)

    * USB_R1 signal should be connected via a 10k resistor to USB_VSSREF.

    * USB_VSSREF signal should be connected directly to ground (Vss).

    Is it ok to leave USB_DM and USB_DP unconnected?

    * It is recommended that USB_DP and USB_DM signals should both be tied to ground (VSS). I'll try to find out if there are consequences for not grounding them.

    Is it ok to leave USB_VBUS unconnected?

    * USB_VBUS signal should be connected to ground (VSS).

    The USB_VBUS is not fault-tolerant. It must not see a voltage when the device is off. There are leakage paths from VBUS to USB_3.3V, which might make DP/DM toggle if a voltage is applied to USB_VBUS. I'll try to find otu if there are consequences for not grounding VBUS.

    =-=-

    The general recommendations for terminating USB when it is not used are below.

    When the USB peripheral is not used,

    * USB_VDDOSC should be connected to ground (VSS).

    * USB_VBUS signal should be connected to ground (VSS).

    * USB_DP and USB_DM signals should both be tied to ground (VSS).

    * USB_R1 signal should be connected via a 10k resistor to USB_VSSREF.

    * USB_VSSREF signal should be connected directly to ground (Vss).

    * USB_VDDA3P3 signal should be connected to ground (VSS).

    * USB_VDDA1P3 signal should be connected to ground (VSS).

    * USB_VDD1P3 signal should be connected to ground (VSS).

    * USB_VDDPLL signal should be connected to ground (VSS).

    * USB_LDOO must be left floating.

    * USB_MXI should be connected directly to ground (VSS).

    * USB_MXO should be left unconnected.

    * USB_VSSOSC signal is connected to board ground (VSS).

    * Tie CLK_SEL high: If CLK_SEL=1, the USB oscillator is disabled at reset and the CLKIN pin is used the source for the system clock generator. In this configuration (CLK_SEL=1), the USB oscillator can be enabled or disabled via software. When CLK_SEL is 1 at reset, the USB LDO is disabled.

    In software,

    * Keep USB LDO disabled by keeping LDOCNTL[USB_LDO_EN] = 0 (it resets to 0 if CLK_SEL = 1)

    * Keep USB OSC disabled by keeping USBSCR [USBOSCDIS] = 1 (it resets to 1 if CLK_SEL = 1)

    * Clock gate the USB peripheral PCGCR2[USBCG]

    These recommendations were pulled form the C5517 Datasheet and cross-checked against the TMS320C5505/15/35/45 Schematic Checklist (SPRAC31, to be released this month) - the USB section applies to C5517 also since its the same USB module.

    Regards,
    Mark

  • 0 in reply to Mark M
    Intellectual 420 points
    Thanks Mark, this was helpful.

    Regards.