PIC18F2455_07 Datasheet, PDF (169/430 Pages) Microchip Technology – 28/40/44-Pin, High Performance, Enhanced Flash, USB Microcontrollers with nanoWatt Technology

PIC18F2455/2550/4455/4550
TABLE 17-1:
VPO VMO
0
0
0
1
1
0
1
1
DIFFERENTIAL OUTPUTS TO
TRANSCEIVER
Bus State
Single-Ended Zero
Differential ‘0’
Differential ‘1’
Illegal Condition
TABLE 17-2: SINGLE-ENDED INPUTS
FROM TRANSCEIVER
VP VM
Bus State
0
0
0
1
1
0
1
1
Single-Ended Zero
Low Speed
High Speed
Error
The UOE signal toggles the state of the external trans-
ceiver. This line is pulled low by the device to enable
the transmission of data from the SIE to an external
device.
17.2.2.3 Internal Pull-up Resistors
The PIC18FX455/X550 devices have built-in pull-up
resistors designed to meet the requirements for
low-speed and full-speed USB. The UPUEN bit
(UCFG<4>) enables the internal pull-ups. Figure 17-1
shows the pull-ups and their control.
17.2.2.4 External Pull-up Resistors
External pull-up may also be used. The VUSB pin may be
used to pull up D+ or D-. The pull-up resistor must be
1.5 kΩ (±5%) as required by the USB specifications.
Figure 17-3 shows an example.
FIGURE 17-3:
PIC^®
Microcontroller
EXTERNAL CIRCUITRY
Host
Controller/HUB
VUSB
1.5 kΩ
D+
D-
Note: The above setting shows a typical connection
for a full-speed configuration using an on-chip
regulator and an external pull-up resistor.
17.2.2.5 Ping-Pong Buffer Configuration
The usage of ping-pong buffers is configured using the
PPB1:PPB0 bits. Refer to Section 17.4.4 “Ping-Pong
Buffering” for a complete explanation of the ping-pong
buffers.
17.2.2.6 USB Output Enable Monitor
The USB OE monitor provides indication as to whether
the SIE is listening to the bus or actively driving the bus.
This is enabled by default when using an external
transceiver or when UCFG<6> = 1.
The USB OE monitoring is useful for initial system
debugging, as well as scope triggering during eye
pattern generation tests.
17.2.2.7 Eye Pattern Test Enable
An automatic eye pattern test can be generated by the
module when the UCFG<7> bit is set. The eye pattern
output will be observable based on module settings,
meaning that the user is first responsible for configuring
the SIE clock settings, pull-up resistor and Transceiver
mode. In addition, the module has to be enabled.
Once UTEYE is set, the module emulates a switch from
a receive to transmit state and will start transmitting a
J-K-J-K bit sequence (K-J-K-J for full speed). The
sequence will be repeated indefinitely while the Eye
Pattern Test mode is enabled.
Note that this bit should never be set while the module
is connected to an actual USB system. This test mode
is intended for board verification to aid with USB certi-
fication tests. It is intended to show a system developer
the noise integrity of the USB signals which can be
affected by board traces, impedance mismatches and
proximity to other system components. It does not
properly test the transition from a receive to a transmit
state. Although the eye pattern is not meant to replace
the more complex USB certification test, it should aid
during first order system debugging.
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 is enabled by default and can be disabled
through the VREGEN Configuration bit. When enabled,
the voltage is visible on pin VUSB. When the regulator
is disabled, a 3.3V source must be provided through
the VUSB pin for the internal transceiver. If the internal
transceiver is disabled, VUSB is not used.
Note 1: Do not enable the internal regulator if an
external regulator is connected to VUSB.
2: VDD must be greater than VUSB at all
times, even with the regulator disabled.
© 2007 Microchip Technology Inc.
Preliminary
DS39632D-page 167