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SN74HC245: SN74HC245 used for extend SPI communication distance in LED display

Betty Guo
Expert 7300 points
Part Number: SN74HC245
Other Parts Discussed in Thread: TLC5955

Hi,

we'd like use SN74HC245 to extend the SPI communicating distance in LED display, and want to know that how long the distance SN74HC345 can handle.

customer use SPI interface to drive 7 pes cascading TLC5955 and the max distance is 1.5 metre. Can the SN74HC245 output drive 7 pes TLC5955 SCLK, GLCK, SDIN and LAT with 1.5 metre distance?

Meanwhile, is there any consideration about SPI communicating wires? which type wires is suitable for SPI? customer use Dupont wires now and SPI signals are very unstable.

could you help on it?

thank you.

  • 0
    TI__Guru* 96681 points

    Hi Betty,

    The load applied to an electronic circuit is not measured in distance - it is measured in inductance, capacitance, and resistance -- the combination of which is referred to as impedance.

    The limiting factor for the output of the SN74HC245 is the current required at the output. From the datasheet, the absolute maximum current rating (before being damaged) at each output is 35 mA, while the typical output current is 7.8 mA. Note that the total current cannot exceed 70mA, so if all 8 outputs are driving at the same time, each can only supply 8.75 mA before exceeding the rated maximum current.

    A good rule of thumb is to keep the capacitance at each output less than 70pF. This will generally prevent problems with signal integrity.

    As for the type of wire to use. In the signal transmission world, a 'wire' is referred to as a 'transmission line' and it has a few important properties:

    Z0 := characteristic impedance

    C := capacitance per unit length of line (typically pF per meter)

    L := inductance per unit length of line (typically nH per meter)

    These three properties are related by this simplified equation: Z0 = sqrt( L / C )

    ** There are also characteristics for resistance and conductance between the wires, however I am assuming that these values are very small for the sake of this analysis. **

    So, for a 50 ohm characteristic impedance transmission line with 101.05 pF/m capacitance, we can easily calculate the inductance per meter as:

    L = C * Z02 = 252.625 nH

    This gives us a simplified lumped element model of the 1.5m transmission line like this:

    You can see that the capacitive load is huge for a 1.5m length of 50 ohm transmission line (RG-58), however transmission lines have a very interesting property -- if the input is correctly matched to the characteristic impedance, the output will look pretty good.  Here's an example showing a well match (top) and a poorly matched (bottom) transmission line:

    I should note that this is a first level analysis, and transmission lines are a very complex subject. I'm leaving out at least 2 full college classes worth of information since I can't really put all that into a single post on this forum.

    The key is to impedance match the circuit, keep capacitance low.