4 replies to this topic

[iced98lx]

Using this screwshield: http://www.ebay.com/...5-/271334265468/ http://proto-pic.co.uk/protoscrewshield/

 

It has an LED hooked up to D13 (I can blink it etc with no issue) but I need to use D13 for SPI as I want to set up a chain of shift registers. Is having the LED on the shield going to mess up my SPI usage?

 

Thanks!

 

Chris

[Mario Vernari]

Hello Chris, and have a happy new year, as well as your family.

 

The led shouldn't be an issue, because the current flowing is low. I'd recommend to keep the SPI speed relatively low, let's say 1-2 MHz max. Also try to keep the SPI wiring short (6 inches max) and, possibly, of the same length (SCLK, MISO, MOSI, SS).

The general rule is: the slower the signals are, the easier will be everything work fine.

 

Have a nice holidays.

Mario

[iced98lx]

Hello Chris, and have a happy new year, as well as your family.

 

The led shouldn't be an issue, because the current flowing is low. I'd recommend to keep the SPI speed relatively low, let's say 1-2 MHz max. Also try to keep the SPI wiring short (6 inches max) and, possibly, of the same length (SCLK, MISO, MOSI, SS).

The general rule is: the slower the signals are, the easier will be everything work fine.

 

Have a nice holidays.

Mario

 

Mario-

   I hope you and yours have a happy new year, and had a wonderful holiday season as well, thank you for the kind wishes.

 

Thank you also for confirming my suspicions on the LED. Based on my application i have no need for fast reacting control of my shift register, slower is more than fine. I'll do some testing and let you know how it turns out.

[Mario Vernari]

Very appreciated, thanks.

 

Just a small note more, although maybe you won't face this case.

You know, the SPI must be "opened" (enabled), and of course can be "closed". When it's closed, the D13 pin (as well as the others involved) can be used as a normal I/O. That is, whenever you set that pin as "input-with-pullup", the led+res act instead as a pull-down. Thus, you should "avoid" this (let's call) collision, because the resulting (floating) voltage may be at an intermediate level, which results in an unstable read.

 

Again, just an alert than a concrete issue. However, if the led+res were connected to the I2C pin that would have been a very annoying trouble, because I2C leverage the pull-up resistors.

 

Hope it's clear.

Cheers

[iced98lx]

Won't be using the pin for anything but SPI but I will keep that in mind. Worst case scenario I modify the shield and remove the LED / Resistor.