I am wanting to drive two 74HCT541 octal buffers with the same 8 signals that consists of four SPI channels (Clk and Data) from a microprocessor in SPI master mode. I need to drive 16 SPI slaves, 8 at one time then the other 8.

So I thought I could use the 8 signals and go into two 74HCT541 chips and enable the first chip and send data to first 8 slaves then disble first chip and enable the second to send to the next 8 slaves.

My concern was that the high impedance output when disabled may make the slaves behave wrong do to maybe noise. So I thought of adding a 1.5K pulldown resister to each output to force the output lines low when the 74HCT541 are disabled. A 1.5K resister would make each output source 3.3ma which is less then the 6ma max. I dont each slave would sync more then the 2.7ma remaining because they will be the only device connected to each output. I could go higher on the resister but I am concerned of the signals integrity.

The speed of the data on SPI is no more then 2Mbps.

Does this seem acceptable or way out of the norm.



74HCT541 Spec Sheet
http://www.mouser.com/ProductDetail...4HCT541N/?qs=sGAEpiMZZMtYFXwiBRPs08bRhzy/ph7V


Thanks for your feedback.
David

 

I have never seen pulldown resistors on tristate devices. unless you have an extreme noise problem, they should not be necessary. if you do have an extreme noise problem, you should fix that first.

 

decided to use a latch instead and use the latch enable to gate the input.

http://www.mouser.com/Search/Produc...3Nvirtualkey59500000virtualkey595-SN74HCT573N