Hi,
I am trying to add a LCD with 24-bit RGB interface and an external SDRAM(IS42S32400F, a variant of the chip used in F746Disco board) to our STM32F767BGTx based product. So far I have read AN4661(Getting started with STM32F7 Series MCU hardware development) and several posts on this forum and on stackoverflow as well. Before I start to route the PCB, I have a few questions hopefully community members can help clarify:
1) How to route FMC_SDCLK trace? AN4661 states that we should "Avoid using serpentine routing". My understanding is that there are only three basic patterns for length tuning: accordion, trombone, sawtooth. Does this mean we can only use sawtooth pattern on FMC_SDCLK trace? Because the other two patterns both involve back and forth (serpentine).
2) What does "±10mm within data/address group" mean? Does that mean the shortest trace within a group can be as much as 20mm shorter than the longest trace in the group?
3) Whether length matching is necessary when all traces are short(all traces are shorter than 35mm)? One post on stackoverflow states that when traces are shorter than 2inch and at 100MHz clock, length matching doesn't really matter.
4) Are LCD controller data and clock traces considered high speed traces and therefore require special treatment on length matching, impedance control? AN4661 doesn't seem to mention RGB traces at all, some article only consider traces with clock above 50MHz high speed traces. LCD controller work at only 20MHz. So I tend to think RGB traces should be categorized as low speed ones and can/should go to the inner low speed layer on a PCB with 6 layer stackup. However, some other articles including one referenced by another post on this forum states that 50Ω impedance control and length matching are required.
In a ideal world, we would like to control impedance of both RGB and FMC traces, and length-match them as close as possible. But in a real world we are often constrained by board space, and cost considerations etc. So dear community members, please help clarify these questions so we can avoid costly PCB iterations.
Thanks,
ZL
