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7) Preamble Packet
The preamble packet is used to inform downstream hubs that a low speed packet is coming.
This allows the low speed drivers to be enabled by all ports connected to low speed devices.
This is repeated for all packets within the transaction and not just preceding the token packet.
This procedure applies only to packets from the host and not packets received from the device.
The preamble packet, like the handshake packet, only includes the 4 bit PID and 4 bit inverted
PID, except that it is not terminated by an EOP. Instead, the bus is driven idle for 4 FS clock,
then the data rate changes to 1.5 MHz and the sync pattern of the LS packet is begun
immediately. The LS packets are otherwise as described in the previous sections.
Packet
Byte
Bit 7
Bit 0
0
1
2
Sync Pattern[7:0]
~PID[3:0]
Change to LS data Rate and begin LS
token, data, or handshake packet as
defined in the above sections.
Idle
PID[3:0]
Figure 18.12 Preamble Packet Format
Summary of Contents for HD64465
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