Maximum trace lenght for 50MHz SPI communication

Hi Ohashi-san, 

This sounds like a general transmission-line problem to me. We need some specifics from the AM2x MCU in question. My answer would be: 

1. Find out what the 50 MHz SPI clock and data bandwidth requirements are and calculate the losses on a PCB transmission line at this loss (db/inch). Bandwidth of these digital signals is mostly a function of the edge-rate on our AM2x 3.3V I/O, this can be from 350-500pS if it is lightly loaded. You can get these edge-rates from our IBIS models [1].

• Assuming a worse-case 350ps edge rate, this translates to about 1.4 GHz bandwidth for any of the digital signals on our devices [2]. 
• You can change this by adding, overdamped, series termination or additional capacitance that will slow down the edge-rates even more [3]. This has the added benefit of also decreasing EMI radiation/susceptibility. 
• Since transmission lines on PCB are lossy and loss is proportional to frequency, the higher the bandwidth of the signal, the more loss/inch it will see
• So decreasing bandwidth with termination resistors is a good idea if you have longer runs

2. Next, you can calculate the transmission line losses at the bandwidth of the signal and pick a loss threshold (attenuation) margin you are comfortable with.

• The AM2x 3.3V LVCMOS I/O are all +/-5% tolerant, so assuming a nominal, well de-coupled power supply (negligible ripple) using maybe +/-2.5% is a reasonable margin.
• So we can plan on 82.5mV drop or 20 * log ((3.3-0.0825)/3.3) = -0.219 dB losses. If you use the full -5% spec, then -0.445 dB.

3. Then, using either a closed form equation estimate/tool like [4], or a 3D EM simulation, you can calculate the losses/inch of the transmission line in question.

• If you use the LP-AM263 top layer 5.3 mil wide traces, on 3.5 mil Iteq IT 180A pre-preg, you get about 0.137dB/inch loss @ 1.4GHz, or 7.299 inches/dB
• 7.299 inches/dB x 0.219 dB  = 1.6 inches
• If you decrease the edge-rate to 500pS, or 1GHz bandwidth, this increases the length to 8.673 inches/dB x 0.219 dB = 1.9 inches

Further increases in transmission line length are possible by using series termination to slow down the edge-rate further, decrease the max operating bandwidth, and therefore operating in a lower frequency, less lossy region of the transmission-line. Also, higher speed substrates with better dielectric constants and loss tangents will decrease the losses across frequency as well. 

[1] AM263x Sitara IBIS Model https://www.ti.com/lit/zip/sprm771 

[2] Johnson, Howard and Martin Graham, High-Speed Digital Design: A Handbook of Black Magic, Prentice Hall, 1993. Summarized here: https://eepower.com/technical-articles/understanding-bandwidth-requirements-when-measuring-switching-characteristics-in-power-electronic-applications/# 

[3] TI Precision Labs "PCB trace impedance matching" https://www.youtube.com/watch?v=DKxDCO1kLEs 

[4] Wcalc: a tool for the analysis and synthesis of transmission line structures and related components https://wcalc.sourceforge.net/