All receive channels in this macro family incorporate an adaptive equalizer, which can compensate for channel insertion loss by attenuating the low frequency components with respect to the high frequency components of the signal, thereby reducing inter-symbol interference.
The equalizer can be configured through the
Portx_EQ bits of the Equalizer Control Register. Table 6-3 summarizes the options, which are:
- No adaptive equalization. The equalizer provides a flat response at the maximum gain. This setting may be appropriate if jitter at the receiver occurs predominantly as a result of crosstalk rather than frequency dependent loss.
- Fully adaptive equalization. Both the low frequency gain and zero position of the equalizer are determined algorithmically by analyzing the data patterns and transition positions in the received data. FTC refers to the algorithm that controls the zero position. In the FTC normal mode, the zero is decreased in frequency when more equalization is needed; in the FTC reversed mode, the zero is increased in frequency when more equalization is needed. The fully adaptive with FTC reversed setting should be used for most applications.
- Hold. The equalizer state is held at its current gain level and zero point.
- Initialize. The equalizer is initialized to a mid-point gain level, with the zero set to a frequency appropriate for the receiver data rate.
- Partially adaptive equalization. The low frequency gain of the equalizer is determined algorithmically by analyzing the data patterns and transition positions in the received data. The zero position is fixed in one of eight zero positions. For any given application, the optimal setting is a function of the loss characteristics of the channel and the spectral density of the signal as well as the data rate, which means it is not possible to identify the best setting by data rate alone, although generally speaking, the lower the line rate, the lower the zero frequency that will be required.
When enabled, the
receiver equalization logic analyzes data patterns and transition times to determine
whether the low frequency gain of the equalizer should be increased or decreased.
For the fully adaptive setting (EQ = 0001), if the low frequency gain
reaches the minimum value, the zero frequency is then reduced. Likewise, if it
reaches the maximum value, the zero frequency is then increased.
The decision logic is implemented as a voting algorithm with a relatively long analysis interval. The slow time constant that results reduces the probability of incorrect decisions but allows the equalizer to compensate for the relatively stable response of the channel.
Table 6-3 Receiver Equalizer Configuration| EQ VALUE | AMPLITUDE REDUCTION |
|---|
| LOW-FREQUENCY GAIN | ZERO FREQUENCY |
|---|
| 0000 | Maximum | - |
| 0001 | Fully Adaptive, FTC Normal |
| 0010 | Fully Adaptive, FTC Reversed |
| 0011 | Hold |
| 0100 | Initialize |
| 0101 |
| 0110 |
| 0111 |
| 1000 | Partially Adaptive | 365 MHz |
| 1001 | 275 MHz |
| 1010 | 195 MHz |
| 1011 | 140 MHz |
| 1100 | 105 MHz |
| 1101 | 75 MHz |
| 1110 | 55 MHz |
| 1111 | 50 MHz |