I noticed on the EZ Kit eval board for the ADSP-21489 that there are 22 ohm resistors on nearly every pin of the DSP. Are these always needed or is this to reduce the ringing on long trace lengths and external buses ?
In addition to this, it is likely that the 21262 and 21369 EZ-KIT Lites - being older boards - were designed before we started ensuring that all signals were terminated with resistors. I believe this is a relatively recent design decision that would not have been retrofitted onto older designs.
Could you please elaborate on what do you mean by “ballast resistors”?
Kindly provide us some more information as to which resistor or a group of resistors you are referring to so that it will help us to locate these resistors on the ADSP-21489 EZ-Kit board.
On the schematic for the ADSP-21489 EZ-BOARD there are 22 ohm resistors connected to nearly every pin on the Sharc processor.
Sheets 2 and 4 illustrate this.
The 22 Ohm resistors are series terminating resistors. We recommend the customers to add the resistors on all pins as is done in the EZ kit to avoid any reflections and ringing specially on memory lines.
I got some more inputs from the EZ-Board design team internally regarding your query. Kindly see my reply below:
These resistors are provided on the EZ-Kit since the DAI and DPI signal go all over the EZ-KIT. They also are used on the SHARC Audio EZ-Extender as well. The best way to ensure that it will be needed or not, is to model the signal path using a high speed signal analysis tool. You can make use of a high end oscilloscope to ensure that the signal look the way they should.
The resistors help with impedance mismatch and reflection due to long traces. If the receiver sees higher than the Vin Max they could potentially have a failure over time. If a long trace causes a switching point in the middle of the rise or fall time, this could result in the part seeing false highs or lows.
To be safe, we would recommend putting 22Ohms or 0Ohms. However, if you concerned about the cost and area sensitive, then you need to simulate and see what works best for you.
I noticed that the EZ kits for the ADSP-21262 and 21369 do not have these resistors. Is this because of the shorter trace lengths ?? In my app I will have fairly short trace lengths to the peripherals and no external memory. I suppose it won't matter if I add the resistors to be sure.
I am not sure whether I will be able to answer this properly. But as far as my understanding is concerned, I think that since, for the address and data lines there are many loads on the EZ-Kit (i.e. SDRAM, flash, SRAM, Sharc USB audio extender etc.), so it is always better to have series terminators.
In general if the room and cost allow for it, it is better to have series terminators since once the boards are back we have more flexibility to look at the signal integrity of the signal and change the values based on scope measurement. Hence to be on the safe side, we recommend putting 22ohm or 0 ohm resistors.
Also as a general rule, if the net is longer than 5-6” and there are multiple loads, adding series terminators helps in reducing any reflections that may occur due to mismatch in line impedance.
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