68HC705JB3 datasheet(84/134 Pages) FREESCALE

84 / 134 page

GENERAL RELEASE SPECIFICATION

November 5, 1998

MOTOROLA

UNIVERSAL SERIAL BUS MODULE

MC68HC05JB3

10-16

REV 1

10.4.4.2 Bit Stufﬁng

In order to ensure adequate signal transitions, bit stufﬁng is employed by the

transmitting device when sending a packet on the USB (see Figure 10-18 and

Figure 10-19). A 0 is inserted after every six consecutive 1’s in the data stream

before the data is NRZI encoded to force a transition in the NRZI data stream.

This gives the receiver logic a data transition at least once every seven bit times to

guarantee the data and clock lock. The receiver must decode the NRZI data,

recognize the stuffed bits, and discard them. Bit stufﬁng is enabled beginning with

the Sync Pattern and throughout the entire transmission. The data “one” that ends

the Sync Pattern is counted as the ﬁrst one in a sequence. Bit stufﬁng is always

enforced, without exception. If required by the bit stufﬁng rules, a zero bit will be

inserted even if it is the last bit before the end-of-packet (EOP) signal.

Figure 10-18. Bit Stufﬁng

IDLE

NRZI

ENCODED

DATA

BIT

STUFFED

DATA

RAW

DATA

STUFFED BIT

SYNC PATTERN

PACKET DATA

SYNC PATTERN

PACKET DATA

SYNC PATTERN

SIX ONES

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