PIC18F2455_09 Datasheet, PDF (172/438 Pages) Microchip Technology – 28/40/44-Pin,High-Performance,Enhanced Flash,USB Microcontrollers with nanoWatt Technology

PIC18F2455/2550/4455/4550
17.2.2.8 Internal Regulator
The PIC18FX455/X550 devices have a built-in 3.3V reg-
ulator to provide power to the internal transceiver and
provide a source for the internal/external pull-ups. An
external 220 nF (±20%) capacitor is required for stability.
Note:
The drive from VUSB is sufficient to only
drive an external pull-up in addition to the
internal transceiver.
The regulator can be enabled or disabled through the
VREGEN Configuration bit. When enabled, the voltage
is visible on pin VUSB whenever the USBEN bit is also
set. When the regulator is disabled (VREGEN = 0), a
3.3V source must be provided through the VUSB pin for
the internal transceiver.
Note 1: Do not enable the internal regulator if an
external regulator is connected to VUSB.
2: VDD must be equal to or greater than
VUSB at all times, even with the regulator
disabled.
17.2.3 USB STATUS REGISTER (USTAT)
The USB Status register reports the transaction status
within the SIE. When the SIE issues a USB transfer
complete interrupt, USTAT should be read to determine
the status of the transfer. USTAT contains the transfer
endpoint number, direction and Ping-Pong Buffer
Pointer value (if used).
Note:
The data in the USB Status register is valid
only when the TRNIF interrupt flag is
asserted.
The USTAT register is actually a read window into a
four-byte status FIFO, maintained by the SIE. It allows
the microcontroller to process one transfer while the
SIE processes additional endpoints (Figure 17-4).
When the SIE completes using a buffer for reading or
writing data, it updates the USTAT register. If another
USB transfer is performed before a transaction
complete interrupt is serviced, the SIE will store the
status of the next transfer into the status FIFO.
Clearing the transfer complete flag bit, TRNIF, causes
the SIE to advance the FIFO. If the next data in the
FIFO holding register is valid, the SIE will reassert the
interrupt within 5 TCY of clearing TRNIF. If no additional
data is present, TRNIF will remain clear; USTAT data
will no longer be reliable.
Note:
If an endpoint request is received while the
USTAT FIFO is full, the SIE will
automatically issue a NAK back to the
host.
FIGURE 17-4:
USTAT FIFO
USTAT from SIE
4-byte FIFO
for USTAT
Data Bus
Clearing TRNIF
Advances FIFO
DS39632E-page 170
© 2009 Microchip Technology Inc.