mc68194 ON Semiconductor, mc68194 Datasheet - Page 16

• Manufacturer Part Number: mc68194
• Description: Carrier Band Modem Cbm
• Manufacturer: ON Semiconductor

4. During end−of−transmission blanking. See Section 6.2.
as a training signal as described in the clock recovery
section. After the PLL has achieved lock, the recovered
clock at this point may be in phase or 180° out of phase with
the bit time clock at the sending end. This creates a problem
for RXCLK and the data recovery logic because symbols
would be decoded as the wrong combination of 1/2 bit time
transitions.
situation. If the clock is 180° out of phase, the PAD−IDLE
sequence (ONE, ZERO, ONE, ZERO, ONE, ...) will be
decoded as a sequence of NON−DATA symbols. Refer to
Figure 6−2. In normal data reception, NON−DATA symbols
occur only in pairs; there are never three or more in a row.
Therefore, three or more NON−DATA symbols occurring in
a row indicate that the bit time clock is 180° out of phase and
the bit time clock (RXCLK) must be slipped as shown in
Figure 6−3. The clock frequency and phase have now been
recovered and symbol decode proceeds as described below.
RECEIVED
SIGNAL
The PAD−IDLE at the beginning of a transmission is used
Logic in the data recovery circuitry corrects for this
NON DATA
INDICATOR
DATA
CLOCK
G
PAD−IDLE
SEQUENCE
BIT TIME
CLOCK IN
PHASE
BIT TIME
CLOCK 180°
OUT OF
PHASE
carrier detect in the amplifier, the PLL must lock to the
new reference clock (generated from the data stream).
During the lockup time, recovered data may not be valid.
The data recovery logic forces SILENCE for a fixed
period of time (17−18 bit times).
Figure 6−3. Clock Slip To Bring In Phase With
Figure 6.2 Training Sequence Decoded With
ND2
In−Phase and Out−Of Phase Clocks
BIT TIME
ND1
ONE
ND2
ND2
CLOCK SLIPPED
1/2 BIT TIME
Data Stream
ND1
ZERO
ZERO
ND1
ONE
ONE
ZERO
ND2
ONE
ZERO
ZERO
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6.2 RECEIVER END−OF−TRANSMISSION BLANKING
the end of a transmission and report silence to the MAC for
a period thereafter. This period of silence is referred to as
blanking and must meet the following conditions:
1. Blanking must begin no later than 4 MAC−symbol times
2. Blanking must continue to a point at least 24
last End Delimiter of a transmission (I Bit = 0, see Section
1.3). The CBM reports silence for 32 symbols after the last
symbol of the End Delimiter.
non−IEEE 802.4 applications via the EOTDIS* input.
The IEEE 802.4 requires that the physical layer recognize
The MC68194 provides this function by recognizing the
The blanking function can be disabled for test purposes or
after the last MAC−symbol of the End Delimiter (i.e., the
last End Delimiter of the transmission).
MAC−symbol times but not more than 32 MAC−symbol
times from the last MAC−symbol of the End Delimiter.