Schematic checkup

[rampadc 29]

Hey guys,

 

Can I please get some feedback on a schematic for a USB/Bluetooth project before ordering the PCBs?

 

[gone]

 

The schematic is divided into 4 sections: power and Li-Po charger on the top left, a connector to external keyboard on the top right, MSP430F5502 on the bottom left and Bluetooth module connections near the bottom right. The two connectors down below are for programming the MSP and the Bluetooth module.

 

The power and Li-Po section should allow a 1-cell Li-Po battery to be charged when USB is plugged in at 500mA charge rate. The 5V booster RT9361A steps 3.0V-4.2V of the Li-Po to 5V to power the PS/2 mouse on the external keyboard connected to CONN. This +5V is then fed into the 3.3V LDO RT9030-33GU5 to supply the rest of the circuit.

 

When USB is plugged in, the F5502 should be able to detect the connection and enumerates. I followed TI's recommended USB schematic layout with a BSL button. The resonator used is a 4MHz resonator.

 

The Bluetooth module should work correctly, as it does standalone on another test PCB.

 

What I'm a bit worried about is if the RT9030-33GU5 3.3V output is going to interfere with the F5502's LDO output on pin 42 (VUSB). As for the DVCCx and AVCCx, I don't have any capacitors near them mainly because I don't have much space left on the PCB but I hope that the MSP430 would still work without them. Would it be a problem?

 

I've checked the design a few times but I always manage to find some possible screw-ups a few days after ordering PCBs.

 

Thanks for your help.

[chicken 630]

What's your reasoning behind connecting VUSB with VCC? Do you want the MSP430 to enumerate and run when the on/off switch is turned off? If that's not a requirement I don't think you need VUSB.

 

If you want VUSB to power the system when VBAT is not present, keep in mind that VUSB can only deliver 12mA max. This might not be enough when the MCU is running and the Bluetooth module is transmitting. 

[rampadc 29]

What's your reasoning behind connecting VUSB with VCC? Do you want the MSP430 to enumerate and run when the on/off switch is turned off? If that's not a requirement I don't think you need VUSB.

 

If you want VUSB to power the system when VBAT is not present, keep in mind that VUSB can only deliver 12mA max. This might not be enough when the MCU is running and the Bluetooth module is transmitting. 

 

From MSP430x5xx family user guide on page 1077, it says that the USB interface is powered from VUSB. If PU.0 and PU.1 are used, either as D+ and D- or as GPIO pins, it is necessary that VUSB is powered from either the internal regulator or an external source. So I assumed that it has to be connected to an external 3.3V source for USB applications if the internal LDO can't delivery much current, with the internal LDO turned off in the Descriptor Tool. (Or do I not need this pin connected at all and leave the internal LDO alone to supply power to the MSP?)

The physical layer interface (USB transceiver) is a differential line driver directly powered from VUSB (3.3
V). The line driver is connected to the DP and DM pins, which form the signaling mechanism of the USB
interface.
When the PUSEL bit is set, DP and DM are configured to function as USB drivers controlled by the USB
core logic. When the bit is cleared, these two pins become "Port U", which is a pair of high-current general
purpose I/O pins. In this case, the pins are controlled by the Port U control registers. Port U is powered
from the VUSB rail, separate from the main device DVCC. If these pins are to be used, whether for USB
or general purpose use, it is necessary that VUSB be properly powered from either the internal regulators
or an external source.

It's certainly not a requirement of the MSP to power circuit when the switch is turned to OFF when connected to USB.

[greeeg 460]

4.2 of the App note you linked slaa457a describes part of the internal power management of the USB module. As well as the MSP430F5xxx family guide.

 

@@chicken is right. The VUSB pin is internally connected to the output of an internal 3.3v LDO that is sourced from VBUS. From what you are saying, leave it disconnected.

(By disconnected I mean, only connect a capacitor between it and ground. As done with the V1.8 pin)

 

I am concerned with your lack of decoupling capacitors. Some 100nF's around your board would go along way to reduce transient spikes. Especially since USB is a high speed bus, not having a stable system voltage could lead to some unpredictable hard to trace errors. You mentioned you don't have much room on the PCB, I'd try for atleast a 100nF at every major IC. (MSP, BLE, LDO, DC/DC). That's only 4.

 

If a device has more than 1 Vdd/Vcc pin then it is recommend to have one on each.

[chicken 630]

Or do I not need this pin connected at all and leave the internal LDO alone to supply power to the MSP?

You won't need to connect this pin to power the USB side of the MSP430, that's what the internal LDO takes care of (if enabled). You still need to connect the other VCC pins of the MSP430 to the external LDO to power the rest of the MCU.

[rampadc 29]

Okay, I've squeezed in as many capacitors as I can, surprisingly a lot  , also removed connection to VUSB.

 

BLE_USB_KBD.PDF

 

[greeeg 460]

VUSB is the output from an internal LDO. it still needs to have capacitance on it.

.(By disconnected I mean, only connect a capacitor between it and ground. As done with the V1.8 pin)

 

First part of the USB manual section describes this.