My New RepRap PCB Design - Questions (was Driver related)

It was suggested I should post this in the electronics section, so here it is.

I am in the process of designing my own driver pcb and I am going to use the A4982/A4984 chips in the TSSOP24 package because...
1. It can dissipate more power
2. It is easier to hand solder than QFN
3. It has full protection including short protection

It seems that the Gen6 software is driving all the steppers in 1/8 mode. Is this correct ?

Is there any reason to allow the steps to be changed from 1, 1/2, 1/4, 1/8 or 1/16 steps (A4982 cannot do 1/8 and A4984 cannot do 1/16) ?
If so, by links or by software ?

Is there any requirement to use other than Automatic Mixed Decay mode ?

Does anyone consider it to be an advantage to have a few preselected current limits set by jumper instead of a potentiometer ?
Would 0.5A, 0.75A, 1A, 1.25A, 1.5A, 1.75A, 2A be sufficient ?
Would a user soldered resistor (supplied) to select be preferable ?

I note that only /ENABLE, DIR & STEP are provided to the driver boards. Has anyone wanted /RESET (or /SLEEP) ?
Could /RESET be common to all drivers ?

Edited 1 time(s). Last edit at 06/05/2011 06:24PM by cluso99.

Pots pwn and multi turn pots pwn much more (e.g. 3296 series).

I think all depends on how much applicability range. E.g. step selection would be nice to have maybe for different applications than mendel deposition (routing - milling maybe). Decay same - slow decay maybe for full steps(?).

More or less related, further than the driver itself i would think about a cheap shift registry for the purpose of mending all the lines to mcu, that is to include MS1&2, sleep, enable, reset etc and others if there are more (other ics). Dont know if its a worthy ideea or how workeable it is, but i think it could make a nice feature, that is if it the arduinos can keep up with a change for this. E.g. supposedly if the mcu can select its own step rate etc, the firmware can drop ton of names there are for each step mode rate ... etc. Somehow simplifying the firmware to cover an even bigger diversity of builds. If the support parts are cheap enoguh i think it just might be worth it.

Well I have seen that the Sanguinololu (a mouthful) board has a common enable. This is what I thought could be achieved. Apparently the Sleep and Resets are tied together and not driven by the micro. BTW, this is a nice pcb.

What I do not want to do is overburden the cost.

However, I would like to be able to set the current limit without the complex method of turning a pot. Hence selecting possible fixed resistors. If I resort to the trusty pot then I will use a multiturn one to get accuracy and reliability.

Next, I am unsure if the repraps would benefit by being able to switch the stepping rate on the fly. From what I have read, it is possible once the speed has been ramped up to switch from 1/8 step to 1/4 then to 1/2 and finally to 1 step, and then reduce gradually back down again. You can only switch the stepping rate at a common step position or you will miss a step(s).

I have a few other thoughts as I progress on my pcb layout. I am tempted to just use the Pololu Drivers for now (A4988 only). Normally I would use SMT throughout as I like small boards. Though, in order to keep costs down, many of my planned options have to be added later although the pcb has the circuitry ready for most of these. This means users will have to add parts to the pcb later, so therefore t/hole is required for most. Of course a couple of parts are not available in thole (FT232 chip).


I could work it out but maybe someone knows... How often (time in ms??) does a step get delivered to the x or y axis stepper when moving the fastest?

I think enable is only really used on Z axis which can be disabled by choice. The x and y disabling is not so "safe" because it probably will loose position. Tieing all enables together, i guess it means when either motor moves they all are on and when none moves all are off, which beats the purpose of having the Z isolated "off" during x or y movements.

Dunno if ~5 resistors + dip switch are less of a trouble than a pot. I assume we talk about the amps tuning by the means of reading the voltage change at the pot, so what kind of selector and what happens during a change, in the middle of the switch when no resistor is selected (changes goes through 0v)? I dunno tbh - but it might be a consideration too. Is it there going to be a mini-micro-transient somewhere, is a sudden VREF pass like 3v-0v-3v going to disturb the ic in any way? Or should resort to only change this offline.

I am unclear as if you want to make a motherboard or a stepper driver, or a package of both. I think in either case, the trouble of making something new ought to be justified by some new features. And the first question is what features would those be? Otherwise think it through: if there is nothing new or nothing different, why not going with an existing setup, like Gen7 or Sanguinololu or any other.

The Sanguino uses a common enable. I would not think you would want to disable the Z while X & Y are running as would not the stepper be able to creep without power???

You would not want to modify the current on the fly, just set it up initially. The circuit can be made failsafe (ie allow 2A max) to keep the chip within specs, but of course the motor is not protected. I don't like the pot from a novice setting this point of view.

I am doing a total solution, motherboard included. I think my first pass will use the Pololu A4988 drivers. Maybe later, provided I build in the features, I can update the Driver pcbs to make this more foolproof and simple. My motherboard is quite well advanced.

My solution has a lot of extra features. But for a cheap minimal solution, Sanguino is it. Very elegant solution and cheap.

Power input to the board remains a problem. What do you use... an old PC power supply most likely. So the HDD connector seems best fit, but it is large and cumbersome and you do not want to be plugging it in and out too often or you will crack the pcb connections. I am looking at a Screw Terminal Block that is pluggable as a possible alternative. Any other thoughts?

BTW I have change the title from "My new driver pcb design" as it is not really suitable now.