AM79C978A AMD [Advanced Micro Devices], AM79C978A Datasheet - Page 78

• Manufacturer Part Number: AM79C978A
• Description: Single-Chip 1/10 Mbps PCI Home Networking Controller
• Manufacturer: AMD [Advanced Micro Devices]

Data Symbols
Data symbols encode data for a much higher
transmission rate, and they do not allow collision
detection.
Data Transmit Timing
A data symbol interval begins with the beginning of trans-
mission of a pulse as shown in Figure 41. Transmit Sym-
bol 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 posi-
tion 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 42.
Immediately after the threshold crossing, the receiver dis-
ables any further detection for a period ISBI-3 TICs
(HPR28 ISBI_SLOW or ISBI_FAST) starting with the de-
tection of the pulse peak.
78
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 41. Transmit Data Symbol Timing
Position 0
Figure 42. Receive Symbol Timing
Position 1
Position 1
Transmitter
Receiver
Position n1
Position n1
Am79C978A
n=0-24
Pulse 1
Pulse 1
PULSE_POSITION_0 occurs at a value defined in Table
15 which determines the transmission speed. When a
pulse begins transmission, the previous symbol 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 measures the
elapsed time from the previous pulse. This value is then
placed in the nearest pulse position bin (one of 25) where
pulse position 0 is at 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 15. 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
22399A-44
22399A-45