Signal Integrity : Driver settings

Signal Integrity is a very important concern for any high speed design. The critical design factor lies in how the receiver can analyze the transmitted signal. So, when a signal is transmitted from Driver to Receiver, the transmission path should disturb the signal to least extent. Even the signal that is propagated over the channel between driver and receiver should not interfere with the other signals on the PCB. So, from the driver side, following are the main considerations that need to be taken for a flawless design:

1. Take a proper call on the drive strength that need to be applied from the driver.

The drive strength depends on the load. So, as the load increases, the drive strength must be high. Remember that load is always measured in terms of capacitance. Higher the capacitance more the load. Consider, a scenario where there is a single driver and 2 receivers, the total load can be considered as sum of the load capacitance of the 2 receivers. This can be a scenario with 2 DIMMs connected to a single processor. If sufficient drive strength is not applied from the driver the signal may decay over time and the receiver may not reliably capture the data. Consider a scenario, where the drive strength is too high, the receiver may reliably capture the data in this case but it may cause some other issues like causing unnecessary distortion on the board because of radiation.

Note: Is is always better to use a highest current drive possible from the transmitter but we have to consider that this doesn't affect other routing on the board.

2. Driver impedance versus current

For a design topology between transmitter and receiver has to work reliably, impedance matching between them is the most important thing. So, when you are trying to drive a load, the impedance of the driver is decided by interface voltage divided by current.This gives the output impedance of the driver. So, sometimes when you set the buffer to output specific current, the impedance may have to be matched in your line. So, the requirements and relation of voltage, current and line impedance are to be understood clearly.

3. IBIS models

So, the best way to determine your channel performance is to perform simulations. The simulations are done using best high speed tools available in the market. One of the example is Hyper lynx. there are other tools like Advanced design system (ADS), Sigrity, etc. Choose the tool that you can learn quickly and use it. IBIS model is a file which characterizes the buffer inside your driver. so, check the IBIS model and set your current requirements. 

4. System thresholds

Considering the system thresholds is always a critical factor in determining the performance of your system. Before using the driver and receiver in your system check the VOH,VOL, VIL, VIH and then perform a theoretical calculations to determine if both are compatible. The logic level in a system plays a crucial role. This settings can be done in the ibis model of the driver chipset.

5. Rise time and fall time

The rise time and fall time of the signal propagating between driver and receiver is very important. In the high speed signal theory, the rise time and fall time are critical. The higher the rise time and fall time the faster the signal. For these kind of signal, the slew rate will be high. In the cases where slew rate is too high, there are chances of radiation within the board. So, controlling slew rate is very important.

6. Load current

The load current plays a crucial role. When you power on the system, the load draws a instantaneous current which need to be taken care by the driver. For some interfaces, a pull-up on the line helps to drive the necessary current. But while designing the system the drive setting must be done to accommodate this current.