ISP1362
Single-chip USB OTG controller
Philips Semiconductors
11.4 Host Negotiation Protocol (HNP)
HNP is used to transfer control of the host role between the default host (A-device)
and the default peripheral (B-device) during a session. When the A-device is ready to
give up its role as a host, it will condition the B-device by SetFeature (b_hnp_enable)
and will go into ‘suspend’. If the B-device wants to use the bus at that time, it signals
a ‘disconnect’ to the A-device. Then, the A-device will take the role of a peripheral
and the B-device will take the role of a host.
11.4.1 Sequence of HNP events
The sequence of events for HNP as observed on the USB bus is illustrated in
Figure 17.
A-device
1
6
8
3
B-device
5
2
7
4
DP Composite
004aaa079
DP driven
Legend
Pull-up dominates
Pull-down dominates
Normal bus activity
Fig 17. HNP sequence of events.
As can be seen in Figure 17:
1. The A-device completes using the bus and stops all bus activity (that is,
suspends the bus).
2. The B-device detects that the bus is idle for more than 5 ms and begins HNP by
turning off the pull-up on DP. This allows the bus to discharge to the SE0 state.
3. The A-device detects SE0 on the bus and recognizes this as a request from the
B-device to become a host. The A-device responds by turning on its DP pull-up
within 3 ms of first detecting SE0 on the bus.
4. After waiting for 30 µs to ensure that the DP line is not HIGH because of the
residual effect of the B-device pull-up, the B-device notices that the DP line is
HIGH and the DM line is LOW (that is, J state). This indicates that the A-device
has recognized the HNP request from the B-device. At this point, the B-device
becomes a host and asserts bus reset to start using the bus. The B-device must
assert the bus reset (that is, SE0) within 1 ms of the time that the A-device turns
on its pull-up.
5. When the B-device completes using the bus, it stops all bus activity. Optionally,
the B-device may turn on its DP pull-up at this time.
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© Koninklijke Philips Electronics N.V. 2004. All rights reserved.
Product data
Rev. 03 — 06 January 2004
34 of 150
NXP [ NXP ]
