Cypress Semiconductor FX2LP Información técnica descargar pdf (Pagina 123)

Chapter 6. Power Management Page 6-1

Chapter 6 Power Management

6.1 Introduction

The USB host can suspend a device to put it into a power-down mode. When the USB signals a

SUSPEND operation, the FX2 goes through a sequence of steps to allow the firmware first to turn

off external power-consuming subsystems, and then to enter a low-power mode by turning off the

FX2’s oscillator. Once suspended, the FX2 is awakened either by resumption of USB bus activity

or by assertion of one of its two WAKEUP pins (provided that they’re enabled). This chapter

describes the suspend-resume mechanism.

It is important to understand the distinction between ‘suspend’, ‘resume’, ‘idle’, and ‘wakeup’.

• SUSPEND is a request—indicated by a 3-millisecond “J” state on the USB bus—from the

USB host/hub to the device. This request is usually sent by the host when

enters a low-

power “suspended” state. USB devices are required to enter a low power state in response

to this request.

The FX2 also provides a register called SUSPEND; writing any value to it will allow the

FX2 to enter the suspended state even when a SUSPEND condition doesn’t exist on the

bus.

• RESUME is a signal from the device to the host, requesting that the host be taken out of its

low-power “suspended” mode. RESUME can be signaled only by a USB device that has

reported (via its Configuration Descriptor) that it supports this “remote wakeup” feature,

and only if the host has enabled remote wakeup from that device.

• Idle is an FX2 low-power state. FX2 firmware initiates this mode by setting bit 0 of the

PCON (Power Control) register. To meet the stringent USB suspend current specification,

the FX2’s oscillator must be stopped; after the PCON.0 bit is set, the oscillator will stop if a)

a SUSPEND condition exists on the bus or the SUSPEND register has been written to,

and b) the two WAKEUP pins are either disabled or false. The FX2 exits the Idle state

when it receives a Wakeup Interrupt.

• Wakeup is the mechanism which restarts the FX2 oscillator and asserts an interrupt to

force the FX2 to exit the Idle state and resume code execution. The FX2 recognizes three

wakeup sources: one from the USB itself (when bus activity resumes) and two from device

pins (WAKEUP and WU2).

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Cypress Semiconductor FX2LP Información técnica Pagina 123