AM79C978 Advanced Micro Devices, AM79C978 Datasheet - Page 79

• Manufacturer Part Number: AM79C978
• Description: Single-Chip 1/10 Mbps PCI Home Networking Controller
• Manufacturer: Advanced Micro Devices

Data Symbols
Data symbols encode data for a much higher transmis-
sion rate, and they do not allow collision detection.
Data Transmit Timing
A data symbol interval begins with the beginning of
transmission of a pulse as shown in Figure 40. Trans-
mit Symbol timing (in TICS) is measured from this point
(TIC = 0).
Depending on the data code, the next pulse may begin
at any PULSE_POSITION_N where N = 0 to 24. Each
position is separated from the previous one by one TIC.
Data Receive Timing
The incoming waveform is formed from the transmitted
pulse. The receiver detects the point at which the enve-
lope of the received waveform crosses a set threshold.
See Figure 41.
Immediately after the threshold crossing, the receiver
disables any further detection for a period ISBI-3 TICs
(HPR28 ISBI_SLOW or ISBI_FAST) starting with the
detection of the pulse peak.
Begin of receive
Blanking interval
Data slice
threshold
START_TX_PULSE
TIC=0
Pulse 0
Pulse 0
END_TX_PULSE
time
Detected Envelope
Data Blanking interval (DISBI)
END_DATA_BLANK
PULSE_POSITION_0
time
Symbol 1
Symbol 1
Figure 40. Transmit Data Symbol Timing
Position 0
Figure 41. Receive Symbol Timing
Position 1
Position 1
Transmitter
Receiver
Position n1
Position n1
n=0-24
Am79C978
Pulse 1
Pulse 1
PULSE_POSITION_0 occurs at a value defined in
Table 13 which determines the transmission speed.
When a pulse begins transmission, the previous sym-
bol interval ends and a new one begins immediately.
The receiver is then re-enabled for pulse detection.
Upon reception of the next pulse, the receiver mea-
sures the elapsed time from the previous pulse. This
value is then placed in the nearest pulse position bin
( o n e o f 2 5 ) w h e r e p u l s e p o s i t i o n 0 i s a t
PULSE_POSITION_0 and each subsequent position is
spaced one TIC from the previous one as defined in the
Data Transmit Timing section. Data symbol intervals
are therefore variable and depend on the encoded
data.
Speed Setting
HIGH_SPEED
LOW_SPEED
Table 13. Blanking Interval Speed Settings
Position 0
Position 0
Symbol 2
Symbol 2
Nominal Data
1 TIC
0.7 Mbps
1.0 Mbps
Position 1
Position 1
Rate
PULSE_POSITION_0
Position n2
Position n2
Pulse 2
Pulse 2
(in TICs)
Value
44
28
22206B-44
22206B-45
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