PIC18F2450/4450
TABLE 14-1: DIFFERENTIAL OUTPUTS TO
TRANSCEIVER
14.2.2.5
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.
VPO VMO
Bus State
0
0
1
1
0
1
0
1
Single-Ended Zero
Differential ‘0’
The USB OE monitoring is useful for initial system
debugging, as well as scope triggering during eye
pattern generation tests.
Differential ‘1’
Illegal Condition
14.2.2.6
Eye Pattern Test Enable
TABLE 14-2: SINGLE-ENDED INPUTS
FROM TRANSCEIVER
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.
VP
VM
Bus State
0
0
1
1
0
1
0
1
Single-Ended Zero
Low Speed
High Speed
Error
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.
The UOE signal toggles the state of the external
transceiver. This line is pulled low by the device to
enable the transmission of data from the SIE to an
external device.
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
certification 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.
14.2.2.3
Pull-up Resistors
The PIC18F2450/4450 devices require an external pull-
up resistor to meet the requirements for low-speed and
full-speed USB. Either an external 3.3V supply or 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 14-3 shows an example with the
VUSB pin.
FIGURE 14-3:
EXTERNAL CIRCUITRY
14.2.2.7
Internal Regulator
PIC®
Microcontroller
Host
Controller/HUB
The PIC18F2450/4450 devices have a built-in 3.3V
regulator to provide power to the internal transceiver and
provide a source for the external pull-ups. An external
220 nF ( 20%) capacitor is required for stability.
VUSB
Note:
The drive from VUSB is sufficient to only
drive an external pull-up in addition to the
internal transceiver.
1.5 kΩ
D+
D-
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: The above setting shows a typical connection
for a full-speed configuration using an on-chip
regulator and an external pull-up resistor.
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
14.2.2.4
Ping-Pong Buffer Configuration
times, even with the regulator disabled.
The usage of ping-pong buffers is configured using the
PPB1:PPB0 bits. Refer to Section 14.4.4 “Ping-Pong
Buffering” for a complete explanation of the ping-pong
buffers.
© 2006 Microchip Technology Inc.
Advance Information
DS39760A-page 133
MICROCHIP [ MICROCHIP ]
