Silicon Laboratories Stepper Machine Specifications download pdf (Page 7)

Attach Event

 Plugging in a USB device to the host root hub or external hub is considered an attach

event. The device has a 1.5 KΩ pull-up resistor to the USB supply (VBUS). Pull-up to D+

signals a full speed device. D- is for low speed devices.

Host or Hub Port

Transceiver

15KΩ 15KΩ Device

Transceiver

D+

D-

VBUS

1.5KΩ

Host or Hub Port

Transceiver

15KΩ 15KΩ Device

Transceiver

D+

D-

VBUS

1.5KΩ

D+

D-

D+

Low Speed Attach Event

Full Speed Attach Event

When no device is connected D+ and D- are at 0 V

So we have a USB device, a host and a USB cable. How does all of this

communication work? We have to first connect everything together. When we

plug the device together with the host it is called an Attach Event (similarly, when

we disconnect it is called a detach event). If you look at the initial condition for the

bus with no devices attached you will notice that the D+ and D- are at the same 0 V

potential because of the 15 Kohm resistors found on the host side. When the cable

is plugged in an endpoint device will provide a pull up resistor on either D+ or D-

depending on its speed capabilities. For a full speed device, the pull-up is attached

to the D+ signal. When the host detects this voltage change it will begin what is

called enumeration process at the full speed rate. If the voltage change is detected

on the D- line, then the enumeration happens at low speed. This attach event is

what signals the host that there is a new device attached to the bus. In MCU devices

the pull-up resistor should be integrated and it is just a matter of setting the bit in a

control register (i.e. the Pull-up Resistor Enable or PREN bit in the USB0XCN

started.

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Silicon Laboratories Stepper Machine Specifications Page 7