AT43301
5
Hub Repeater
The Hub Repeater is responsible for port connectivity setup
and teardown. It also supports exception handling such as
bus fault detection and recovery, and connect/disconnect
detection. Port0 is the root port and is connected to the root
hub or an upstream hub. When a packet is received at
Port0, the AT43301 propagates it to all the enabled down-
stream ports. Conversely, a packet from a downstream port
is transmitted from Port0.
The AT43301 supports downstream port data signaling at
both 1.5 Mb/s and 12 Mb/s. Devices attached to the down-
stream ports are determined to be either full speed or low
speed depending which data line (DP or DM) is pulled high.
If a port is enumerated as low speed, its output buffers
operate at a slew rate of 75-300 ns, and the AT43301 will
not propagate any traffic to that port unless it is prefaced
with a preamble PID. Low speed data following the pream-
ble PID is propagated to both low- and full-speed devices.
The AT43301 will enable low-speed drivers within four full-
speed bit times of the last bit of a preamble PID, and will
disable them at the end of an EOP. The upstream traffic
from all ports is propagated by Port0 using the full speed 4-
20ns slew rate drivers.
All the AT43301 ports independently drive and monitor
their DP and DM pins so that they are able to detect and
generate the ‘J’, ‘K’, and SE0 bus signaling states. Each
hub port has single-ended and differential receivers on its
DP and DM lines. The ports’ I/O buffers comply with the
voltage levels and drive requirements as specified in the
USB Specifications Rev 1.0.
The Hub Repeater implements a frame timer which is timed
by the 12 MHz USB clock and gets reset every time an
SOF token is received from the host.
Serial Interface Engine
The Serial Interface Engine handles the USB communica-
tion protocol. It performs the USB clock/data separation,
the NRZI data encoding/decoding, bit stuffing, CRC gener-
ation and checking, USB packet ID decoding and genera-
tion, and data serialization and de-serialization. The on-
chip phase locked loop generates the high frequency clock
for the clock/data separation circuit.
Power Management
A hub is allowed to draw up to 500 mA of power from the
host or upstream hub. The AT43301’s itself and its external
circuitry typically consume about 24 mA. Therefore, in the
bus-powered mode, 100 mA is available for each of the
hub’s downstream devices. In the self-powered mode, an
external power supply is required which must be capable of
supplying 500 mA per port. The power supplied to the ports
is monitored and controlled by the AT43301.
The AT43301 reports overcurrent on a global basis. The
overcurrent signal, which needs to be detected by an exter-
nal device, is read through the OVC pin. A logic low at OVC
is interpreted as an overcurrent condition. This could be
caused by an overload, or a short circuit, and causes the
AT43301 to set the Over-Current Indicator bit of the Hub
Status Field, wHubStatus, as well as the Over-Current Indi-
cator Change bit of the Hub Change Field, wHubChange.
At the same time, power to the ports is switched off by de-
asserting PWR.
An external device is needed to perform the actual switch-
ing of the ports’ power under control of the AT43301. The
signal to control the external switch is the PWR pin which is
an open drain signal and requires an external pull-up resis-
tor. 47 k
Ω
is a typical value for this resistor. Any type of
suitable switch or device is acceptable. However, the
switch should have a low-voltage drop across it even when
the port absorbs full power. In its simplest form, this switch
can be a high side MOSFET switch. The advantage of
using a MOSFET switch is its very low-voltage drop.
The power control pin, PWR, is asserted only when a Set-
PortFeature[PORT-POWER] request is received from the
host. PWR is de-asserted under the following conditions:
1.
Power up
2.
Reset and initialization
3.
Overcurrent condition
4.
Requested by the host though a ClearPortFea-
ture[PORT_POWER] for ALL the ports
Self-powered Mode
In the self-powered mode, power to the downstream ports
must be supplied by an external power supply. This power
supply must be capable of supplying 500 mA per port or 2A
total with good voltage tolerance and regulation. At full hub
operating power, that is all downstream ports drawing 500
mA each, the minimum voltage at the downstream port
connector must be 4.75V.
The USB specification requires that the voltage drop at the
power switch and board traces be no more than 100 mV. A
good conservative maximum drop at the power switch itself
should be no more than 75 mV. Careful design and selec-
tion of the power switch and PC board layout is required to
meet the specifications. When using a MOSFET switch, its
resistance must be 40 m
Ω
or less under worst case condi-
tions. A suitable MOSFET switch for an AT43301 based
hub is an integrated highside MOSFET switch such as the
Micrel MIC2505.
