68HC705JB4 datasheet(72/134 Pages) FREESCALE

72 / 134 page

GENERAL RELEASE SPECIFICATION

February 24, 1999

MOTOROLA

UNIVERSAL SERIAL BUS MODULE

MC68HC05JB4

10-6

REV 2

10.2.1.4 Endpoint Field (ENDP)

The Endpoint ﬁeld is a four bit number that is used to select a particular endpoint

within a USB device. For the MC68HC05JB4, this will be a binary number

between zero and two inclusive. Any other value will cause the transaction to be

ignored.

10.2.1.5 Cyclic Redundancy Check (CRC)

Cyclic Redundancy Checks are used to verify the address and data stream of a

USB transaction. This ﬁeld is ﬁve bits wide for token packets and sixteen bits wide

for data packets. CRCs are generated in the transmitter and sent on the USB data

lines after both the endpoint ﬁeld and the data ﬁeld. Figure 10-6 shows how the

ﬁve bit CRC value is calculated from the data stream and veriﬁed for the address

and endpoint ﬁelds of a token packet. Figure 10-7 shows how the sixteen bit CRC

value is calculated and either transmitted or veriﬁed for the data packet of a given

transaction.

Figure 10-6. CRC Block Diagram for Address and Endpoint Fields

00 1 0 1

01 1 0 0

next bit

Data Stream

- Update every bit time.

- Reset to ones at SOP

Equal?

Good CRC

Bad CRC

Expected Residual:

Generator Polynomial:

MUX

Freescale Semiconductor, Inc.

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