Cypress Semiconductor FX2LP Technical Information Seite 124

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EZ-USB FX2 Technical Reference Manual
Page 6-2 EZ-USB FX2 Technical Reference Manual v2.1
The FX2 enters and exits its Idle state independent of USB activity; in other words, the FX2 can
enter the Idle state at any time, even when not connected to USB. The Idle state is “hooked into”
the USB SUSPEND-RESUME mechanism using interrupts. An interrupt is automatically gener-
ated when the USB goes inactive for 3 milliseconds; FX2 firmware may respond to that interrupt
by entering the Idle state to reduce power. If the FX2 is in the Idle state, a Wakeup Interrupt is
generated when one of the three Wakeup sources is asserted; the FX2 responds to that interrupt
by exiting the Idle state and resuming code execution.
Once the FX2 is awake, its firmware may send a USB RESUME request by setting the SIGR-
SUME bit in the USBCS register (at 0xE680). Before sending the RESUME request, the device
must have: a) reported remote-wakeup capability in its Configuration Descriptor, and b) been
given permission (via a Set Feature-Remote Wakeup request from the host) to use that remote-
wakeup capability. To be compliant with the USB Specification, firmware should wait 5 millisec-
onds after the wakeup interrupt, set the SIGRSUME bit, wait 10-15 milliseconds, then clear it.
Figure 6-1 illustrates the FX2 logic that implements USB suspend and resume. These operations
are explained in the next sections.
Figure 6-1.
Suspend-Resume Control
PLL
Oscillator
divider
8051
CLKOUT
24 MHz
START
USB RESUME
WAKEUP pin
PCON.0
STOP
USB
"SUSPEND"
Interrupt
No USB activity
for 3 msec.
"RESUME" INT
Signal
Resume
(USBCS.0)
Restart
Delay
WU2 pin
DPEN
WUEN
WU2EN
WUPOL
WU2POL
Write any value to
SUSPEND register
(0xE681)
Resume
Suspend
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