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15.4.8 Suspend/Resume and SRP
This chapter describes different methods of saving power when the USB is suspended. This chapter
discusses the following topics:
• Placing PHY in Low Power Mode Without Entering Suspend (p. 335)
• When the Core is in Host Mode (p. 335)
• When the Core is in Device Mode (p. 336)
• Suspend (p. 336)
• Using EM2 (p. 336)
• Overview of the EM2 Programming Model (p. 336)
• Using EM2 when the Core is in Host Mode (p. 336)
• EM2 when the Core is in Device Mode (p. 339)
• Clock Gating (EM0 and EM1) (p. 341)
• Internal Clock Gating when the Core is in Host Mode (p. 341)
• Internal Clock Gating when the Core is in Device Mode (p. 342)
15.4.8.1 Placing PHY in Low Power Mode Without Entering Suspend
The core can place the PHY in low power mode (the differential receiver is disabled) without entering
suspend.
15.4.8.1.1 When the Core is in Host Mode
Programming flow for the Host Core to put PHY in low power mode
1. To turn off port power, perform write operation to set the following bits in the USB_HPRT register:
• USB_HPRT.PRTPWR = 0;
• USB_HPRT.PRTENA = 0;
2. To put PHY in low power mode, perform read-modify-write operation to set the following bits in the
USB_PCGCCTL register:
• USB_PCGCCTL.STOPPCLK = 1
• USB_PCGCCTL.GATEHCLK = 0
Programming flow for the Host Core to make PHY exit low power mode
If your device is non-SRP capable, the host must implement polling to detect the device connection by
turning on the port and exiting PHY low power mode periodically and checking for connect.
1. To turn on port power, perform write operation to set the following bits in the USB_HPRT register:
• USB_HPRT.PRTPWR = 1
• USB_HPRT.PRTENA = 0
2. To exit PHY low power mode, perform read-modify-write operation to set the following bits in the
USB_PCGCCTL register:
• USB_PCGCCTL.STOPPCLK = 0
• USB_PCGCCTL.STOPHCLK = 0
3. Wait for the USB_HPRT Port Connect Detected (PRTCONNDET) bit to be set and do the enumeration
of the device.
If your device is SRP-capable, when the device initiates SRP request, the Host core asynchronously
detects SRP and the PHY exits low power mode.
1. Wait for Session Request from the device, or New Session Detected Interrupt (SESSREQINT) in the
USB_GINTSTS register.
2. To turn on port power, perform write operation to set the following bits in the USB_HPRT register:
2013-05-08 - Wonder Gecko Family - d0233_Rev0.50
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