bus loading with the Blackfin BF533

Document created by analog-archivist Employee on Feb 23, 2016
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We are looking for information on bus loading with the Blackfin BF533 device.
The goal is to use the emif with 4 external device (Flash / 2xsdram / duart (18
pf on this device));

On the datasheet we found information on bus loading with Sdram of 50 pf: "For
proper SDRAM controller operation, the maximum load capacitance is 50 pF (at
3.3 V) or 30 pF (at 2.5 V) for ADDR19-1, DATA15-0, ABE1-0/SDQM1-0, CLKOUT,
SCKE, SA10, SRAS, SCAS, SWE, and SMS"

We suppose that like other DSP, there is a rule of thumb derating of 6 to 7ns
per 50pf capacitance. To avoid (or not) the use of buffer, we need this
information.

 

The Capacitive Loading section in the ADSP-BF533/2/1 datasheet provides several
figures showing how output rise time varies with capacitance. The delay and
hold specifications given should be derated by a factor derived from these
figures.

The described bus loading (FLASH, 2xSDRAM and DUART) should not require you to
buffer the SDRAM devices, but obviously, it all depends on the capacitive load
of each specific device being used. Please refer to each device's datasheet to
obtain this information so that you can then use the figures provided in the
ADSP-BF533/2/1 datasheet to determine the derating factor. Also, a very
important factor here is the system clock speed used.

As an example, some the ADSP-BF533 STAMP board design includes 4xSDRAMs + FLASH
+ Ethernet Controller and some other devices all connected to the same bus with
no external buffering. But again, this depends on your specific design
requirements (system clock, etc).

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