Content originally posted in LPCWare by Rob65 on Mon Jun 11 11:37:50 MST 2012
Thanks to kayoda for providing that application note.
Too bad that most devices don't include any ESD/EMI on SD card interfaces. Mostly they think/hope/pray that the metal sleeve of the SD slot is already enough protection. There are too few 'engineers' (if you can still call them engineers) that state that once a chip is on the PCB you've already got enough ESD protection ...
For those who don't know: ESD is like those little shocks you get when pulling of a sweater during dry weather or when you tough the doorknob after walking over a carpet with sneakers. Our body works like a capacitor collecting electrical charge as we move around. Whenever we then touch some conductive device, this electric charge will be evenly distributed between two 'bodies'. If the charge is large enough we may feel this and even see a little flash in the dark.
This discharge of our charge into another device is ESD. It's a small current that flows from our body into the device. This discharge may be large enough to damage the device that we touch. Even if we don't feel anything, even if the device still works as expected, this discharge may damage the device. In the LPCXpresso controller a transistor may be damaged a little bit, a diode may be damaged or a wire on the chip may be damaged. The device may still be working but in two weeks up to a number of years this may result in a defective chip.
Compare this to a little bit of wear on a rope: it's still strong but the spot with the wear may continue to grow under stress and in the end the rope will break.
If you look at my schematics, you'll see the USB connector (top left) has a special protection circuit with 5 diodes - this is a very small device with 5 high power diodes to take the ESD pulses and protect the microcontroller.
As an extra security, the metal casing of the USB plus is not directly connected to the ground connection but through a 1 Mohm, 10 nF capacitor pair.
The idea behind this is that any ESD that hits the metal shell is discharged through the capacitor (the capacitor charges) and the 1M resistor discharges the capacitor again.
The idea of not having any ESD protection on the SD card is that this I used a hinge type connector that is embedded inside a device (no user access). The SD card is (ahum) 'factory' mounted and is not used replaceable. And if it was - the user would discharge through the metal hinge before coming in critical range of the data connections (I should have added an R/C combination on the hinge part for minimum safety ...). In fact, the USB connector is the only electrical part that will ever come in close range of the user.
Regarding the pull-up resistors: the LPC1xxx contains internal weak pull up resistors - they seem to work for me ... indeed it is much better (in a real device) to use the external 10k pull ups as on the base board.
Rob
P.s: don't think lightly about ESD. I've seen perfectly good working board suddenly having a malfunctioning peripheral (or worse)
