SAM7S256_14 Datasheet, PDF (504/775 Pages) ATMEL Corporation – ARM-based Flash MCU

35.4 Typical Connection
Figure 35-2. Board Schematic to Interface USB Device Peripheral
5V Bus Monitoring
PIO
27 K
47 K
3V3
PIO
Pullup Control
0: Enable
1: Disable
DDM
DDP
R_EXT
R_EXT
330 K
1.5K
2
1
330 K
3 ^{Type B} 4
Connector
35.4.1 USB Device Transceiver
The USB device transceiver is embedded in the product. A few discrete components are required as follows:
• the application detects all device states as defined in chapter 9 of the USB specification;
– pullup enable/disable
– VBUS monitoring
• to reduce power consumption the host is disconnected
• for line termination.
Pullup enable/disable is done through a MOSFET controlled by a PIO. The pullup is enabled when the PIO drives
a 0. Thus PIO default state to 1 corresponds to a pullup disable. Once the pullup is enabled, the host will force a
device reset 100 ms later. Bus powered devices must connect the pullup within 100 ms.
35.4.2 VBUS Monitoring
VBUS monitoring is required to detect host connection. VBUS monitoring is done using a standard PIO with inter-
nal pullup disabled. When the host is switched off, it should be considered as a disconnect, the pullup must be
disabled in order to prevent powering the host through the pull-up resistor.
When the host is disconnected and the transceiver is enabled, then DDP and DDM are floating. This may lead to
over consumption. A solution is to connect 330 KΩ pulldowns on DP and DM. These pulldowns do not alter DDP
and DDM signal integrity.
A termination serial resistor must be connected to DP and DM. The resistor value is defined in the electrical speci-
fication of the product (R_EXT).
SAM7S Series [DATASHEET]
6175M–ATARM–26-Oct-12
504