Cypress Semiconductor FX2LP Technical Information Seite 132

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EZ-USB FX2 Technical Reference Manual
Page 7-2 EZ-USB FX2 Technical Reference Manual v2.1
7.2 Power-On Reset (POR)
An active-low input pin (RESET) resets the FX2 chip. Note that the term “Power-On Reset”
refers to a reset initiated either by application of power or by assertion of the RESET
pin.
The RESET
pin is normally connected to an external R-C network in order to ensure that, when
power is first applied, the FX2 is held in reset until the operating parameters (Vcc voltage, crystal
frequency, PLL frequency, etc.) stabilize. The recommended values for the R-C network are a 10K
resistor to Vcc and a 1
µ
F capacitor to GND (see Figure 7-1). External logic can force a POR at
any time by pulling the RESET
pin low.
Whenever the RESET
pin is asserted, the USB Core holds the CPU in reset.
The CLKOUT pin, crystal oscillator, and PLL are active as soon as power is applied. Once the
CPU is out of reset, firmware may clear a control bit (CLKOE, CPUCS.1) to inhibit the CLKOUT
output pin for EMI-sensitive applications that do not need this signal.
The CLKOUT signal is active while RESET
is low. When RESET returns high, the activity on the
CLKOUT pin depends on whether or not the FX2 is in the low-power “suspend” state; if it is, CLK-
OUT stops. Resumption of USB bus activity or assertion of the WAKEUP
or WU2 pin (if enabled)
restarts the CLKOUT signal.
The oscillator and PLL are unaffected by the state of the RESET
pin.
Power-on default values for all FX2 register bits are shown in Chapter 15, "Registers". At power-
on reset:
Endpoint data buffers and byte counts are uninitialized.
The CPU clock speed is set to 12 MHz, the CPU is held in reset, and the CLKOUT pin is
active.
All port pins are configured as general-purpose input pins.
USB interrupts are disabled and USB interrupt requests are cleared.
Bulk IN and OUT endpoints are unarmed, and their stall bits are cleared. The FX2 will
NAK IN and OUT tokens while the CPU is reset.
Endpoint toggle bits are cleared to 0.
The RENUM bit is cleared to 0. This means that the Default USB Device, not the firmware,
will respond to USB device requests.
The USB Function Address register is cleared to zero.
The endpoints are configured for the Default USB Device.
Interrupt autovectoring is turned off.
Configuration Zero, Alternate Setting Zero is in effect.
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