In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Zoot wrote:

[quote="Somebody"]// The Backpack requires 125Khz SPI -> which is the slowest rate at which the Arduino's hardware SPI bus can communicate at. [/quote]

125khz SPI -- that's not the frequency of the uController clock, that's the bit rate on clocked data transfers, e.g. SHIFTIN/SHIFTOUT (for SPI) and I2C. The SX/B documentation says that SHIFTIN/SHIFTOUT use a rate of 83khz, independent of uC clock setting. Far below the max data transfer rate of the backpacks. The forum post you pointed me to refers to a controller where the slowest rate available is 125khz, which means that controller is pushing the envelope on what the backpack can handle. Remember too, that with any clocked data transfer, slower will generally be fine, while faster will not. In this case, unless you are using the speedmult parameter on your SHIFTOUT to jack up the transfer rate, I don't think that's it. 

I would look to two places in your current code -- when/where you latch the cascaded shift registers on the backpack, and which rows/columns of your data and/or buffer are being dumped.

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