guitar pedal router stuff again - Page 1
Forum section: electronics / general

author: Dan Moos
author_rank: Frequent Contributor
timestamp: March 23, 2016, 01:20:14 am
content: Ok, for initial experimentation, i have a couple MT8816 8x16 crosspoint switches to play with. I have a different 16x16 chip coming, but want to play with this one first.Prepare for dumb noob questions!Assuming I'm going to treat them as active high, i presume my address, strobe, and data pins all need pulldowns, correct? I have been using 10k for this sorta thing, but know one seems to have a good rule of thumb.Ok, powering the chip. In initial experimentation, I'm just gonna tie a random X switch pin to my ardiunio's 5v, and have a random Y switch pin hooked to an LED with a resistor to ground, and see if I can turn it on . I was gonna put the chip's Vee and Vss to the arduino's GND, and VDD to the ardiuno's 5v. The quiescent current on the chip is crazy low if I read the datasheet right, and one LED set up f0r 5mA isnt gonna add much, so I presume I'm well within the limits of the Arduino's current limits.But obviously when it comes time to do the real thing, the MT8816 is gonna need bigger rails. I'm planning + and minus 10 or more. So now using my bench PS, I would hook Vdd to +10, Vee to -10, and Vss to 0. My question is, with this setup, can I still run the arduino off my USB with all that going on? I presume yes, but it just seems safe to ask. Also, is there any reason not to hook the Arduino's ground to the PS so the whole shebang shares a zero reference. Note, I would NOT be using the green mains earth connection on my PS, so that portion would be floating.Or am I better off running the arduino off the PS also, feeding 10 volts to the Vin pin. If I did that, can I still leave my arduino hooked to USB? Will there be a conflict because it has two power sources, or does it automatically power solely from the Vin pin when it is powered?I know these are dumb questions, but I'm timid of wrecking my arduino and or an 11 dollar CMOS chip!

author: SL4P
author_rank: Super Contributor
timestamp: March 23, 2016, 05:21:47 am
content: Pull-downs are generally not needed, as long as your driving circuit (micro etc) has bipolar outputs (or open collector at the least)... pull-ups - MAY be needed (without looking at the target chip data)Common 0V rail is important, but be really careful that the -10V NEVER gets anywhere near the Arduino!You shouldn't have any problem running the Arduino from the +10V supply, you may want to add a couple of modest electrolytics to smooth the +rail (and the -rail for that matter).The green mains earth can be your friend in audio applications. Keep a connection point where you may provide a single binding connection to ground - which will assist your audio shields etc maintain their shielding. The problem creeps in when you have multiple grounds - forming a loop.

author: Ian.M
author_rank: Super Contributor
timestamp: March 23, 2016, 05:54:30 am
content: I would suggest splitting the resistor for the LED in two and putting half the value either side of the matrix. Less risk of damage if you get something wrong. Keep the LED current low. With a high brightness LED 1mA is plenty, and that also reduces the risk of damaging an $EXPEN$IVE$ chip.

author: Dan Moos
author_rank: Frequent Contributor
timestamp: March 23, 2016, 06:34:00 am
content: Ok, I got it working. I successfully control 2 switch combos in the matrix, i.e., I have two LEDS set up on different xy pairs. All works well. I sorta see why you suggest splitting the current limiters, but had already started with no troubles.The datasheet says the STROBE on the switch chip needs to be high for at least 20ns. I used a delay(1) statement for a 1 mS HIGH, then set it LOW. Is a delay the best way to do that? It worked, but I wonder if that's the real way to time my pulse.My next goal is to put my address and data lines on a shift register to reclaim some I/O.One part of my power question is still unclear. Can I have the Arduino powered from USB at the same time the crosspoint chip is powered from higher rails? I can't see where a problem would be, but just checking.Also, when I do power the Arduino from my bench PS, can I still leave the USB hooked up without a conflict? Again, I presume yes, but want to be safe.

author: Ian.M
author_rank: Super Contributor
timestamp: March 23, 2016, 07:55:18 am
content: Power sequencing issues!What happens if the Arduino is powered but the +/-10V rails aren't, or visa-versa?Its best to power both the Arduino and the crosspoint chip from the same PSU to avoid any risk of damage. As the Arduino will always take power from USB if the DC power in is off, you should add a potential divider from the +10V supply to an analog input so the Arduino can sense if the external power is off and if so, disable its digital outputs so it doesn't try to drive an unpowered matrix chip.

author: obiwanjacobi
author_rank: Super Contributor
timestamp: March 23, 2016, 08:25:27 am
content: Quote from: Dan Moos on March 23, 2016, 06:34:00 amIf you use the Arduino libraries (digitalWrite etc) you do not have to put in a delay for the 20ns pulse. Just write it high and then low immediately. The digitalWrite function has considerable overhead (translating arduino pins to MCU pins for instance) which will delay plenty.You can getsimple 8 channel logic analyzers on ebayfor about 8$. Download the free software from Saleae and you have a cheap and simple instrument to get an idea of what your digital lines are doing. This will help you answer this kind of questions.