NRF52820-QDAA-R datasheet(374/437 Pages) NORDIC

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Peripherals

6.26.5 USB pull-up

The USB pull-up serves two purposes: it indicates to the host that the device is connected to the USB bus,

and it indicates the device's speed capability.

When no pull-up is connected to the USB bus, the host sees both D+ and D- lines low, as they are pulled

down on the host side by 15 kΩ resistors. The device is not seen by the host and hence is in a detached

state, even though it could be physically connected to the host. The USB Specification does not allow any

current drawn on the VBUS in that type of situation.

When a full-speed device connects its 1.5 kΩ pull-up to D+, the host sees the corresponding line high. The

device is then in the attached state. During the enumeration process, the host attempts to determine if

the full-speed device also supports higher speeds and initiates communication with the device to further

identify it. The USBD peripheral implemented in this device supports only full-speed (12 Mbps), and thus

ignores the negotiation for higher speeds in accordance with the USB specification revision 2.0.

This allows the software to control when USB enumeration takes place. It also allows to emulate a physical

disconnect from the USB bus, for instance when re-enumeration is required. USBPULLUP has to be

enabled to allow the USBD to handle USB traffic and generate appropriate events. This forbids the use of

an external pull-up.

Note that disconnecting the pull-up through register USBPULLUP while connected to a host, will result in

both D+ and D- lines to be pulled low by the host's pull-down resistors. However, as mentioned above, this

will also inhibit the generation of the USBRESET event. The pull-up is disabled by default after a chip reset.

The pull-up shall only get connected after USBD has been enabled through register ENABLE. The USB

pull-up value is automatically changed depending on the bus activity, as specified in Resistor ECN which

amends the original USB 2.0 Specification. The user does not have access to this function as it is handled in

hardware.

While they should never be used in normal traffic activity, lines D+ and D- may at any time be forced into

state specified in register DPDMVALUE by the task DPDMDRIVE. The DPDMNODRIVE task stops driving

them, and PHY returns to normal operation.

6.26.6 USB reset

The USB specification defines a USB reset, which is not be confused with a chip reset. The USB reset is a

normal USB bus condition, and is used as part of the enumeration sequence, it does not reset the chip.

The USB reset results from a single-ended low state (SE0) on lines D+/D- for a tUSB,DETRST amount of time.

Only the host is allowed to drive a USB reset condition on the bus. The UBSD peripheral automatically

interprets a SE0 longer than tUSB,DETRST as a USB reset. When the device detects a USB reset and generates

a USBRESET event, the device USB stack and related parts of the application shall re-initialize themselves,

and go back to the default state.

Some of the registers in the USBD peripheral get automatically reset to a known state, in particular all data

endpoints are disabled and the USBADDR reset to 0.

After the device has connected to the USB bus (i.e. after VBUS is applied), the device shall not respond to

any traffic from the time the pull-up is enabled until it has seen a USB reset condition. This is automatically

ensured by the USBD.

After a USB reset, the device shall be fully responsive after at most TRSTRCY (according to chapter 7 in the

USB specification). Software shall take into account this time that takes the hardware to recover from a

USB reset condition.

4463_156 v1.0

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