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Maximum speed of spi

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Dan Mills

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Depends on the timing of the slowest part you are using, check the datasheets and figure out what the fastest you can run things while still meeting timing.

You see anything from a few MHz to 50MHz or more depending on the parts in question.

Regards, Dan.
 

ravindragudi

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Adding to Dan's comments - Even the spacing or length of traces and hence loading on the SPI bus will decide the max clock frequency.
 

andre_teprom

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You see anything from a few MHz to 50MHz or more depending on the parts in question
Devices operating with the conventional standard SPI need pull-up resistors, which makes the bus somewhat "weak". For frequencies above the MHz order, must consider use an electrical standards based on differential pairs.
 

Dan Mills

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Andre,
Thats I2C you are thinking of, which is often limited to a few hundred KHz by the pull ups, SPI has totem pole drivers on all lines and so can run at reasonable speeds.

Regards, Dan.
 

hobbyckts

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Is pull-up resistor is quite necessary in the SPI lines. If CS is active low we can use a pull-up resistor for these. But is it required for remaining lines?
 

FvM

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The original SPI specification has been issued by Motorola and doesn't use any open drain signals. Instead MISO (slave data out) has tristated push-pull drivers, the other (master driven) signals are permanently enabled push-pull. Open drain with pull-up is used in some SPI alike variants, but isn't genuine SPI technique.

Recent fast SPI devices with 50 MHz and higher clock frequency are using a CMOS I/O standard, mostly 3.3V. The bus extension is of course limited with highest speeds, but a few 10 cm can be achieved with suitable design, e.g. using source-side series termination. Differential signalling, as mentioned by andre_teprom, can extend SPI range to many meters, mainly limited by cable and tranceiver delay and respective round-trip time.
 

ads-ee

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Is pull-up resistor is quite necessary in the SPI lines. If CS is active low we can use a pull-up resistor for these. But is it required for remaining lines?
As all SPI lines are driven by totem-pole drivers you don't need pullups on any of the lines.

Also there was never any lower limit frequency specification given for SPI back in the old 68HC microcontroller days. Even the current versions of the part don't specify any lower limit frequency. AFAIK you could run the SPI at 1 Hz.
The maximum frequency is not defined either, except for each individual part. If I recall correctly the original SPI was run a 1 MHz but that wasn't defined by any specification just the documentation for the interface in the part it was used in. You could run SPI at 1 GHz if you could manage the signal integrity and the problems with having to use the SPI clock to run the interface (You probably don't want to try and oversample a 1 GHz SPI interface like most designs do).
 

hobbyckts

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Thanks for the reply with a quite clear explanation. The previous discussions where they were discussing about this question that is the reason why I have asked this.
 

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