I was not talking about protecting the AVR against false resets, but about protecting the rest pin against ESD which can potentially damage the IC. A rare false reset wouldn't be such a big deal anyway since the program will just restart and start reading the buttons again.Originally Posted by no2chem
Unlike all the other port pins, the reset pin doesn't have an internal proctection diode connected to VCC. The reason for this is to allow high voltage programming, which would be impossible if a diode to VCC had been present internally. But since you don't use high voltage programmning and since you have exposed the rest pin to an external ISP circuit, it's a good practice to add an external protection diode to the reset pin as described in AVR042 section 3. All the other port pins has this protection built inside the IC, so it's just a matter of adding the same kind of protection to the rest pin externally.
Just use a standard LL4148 or LL4448 SMD diode in SOD80 housing. These diodes are dirt cheap.
I would prefer a mini-usb connector or no connector at all. Like no2chem said, I dont have 10 micro-usb cables lying around where as I do have mini-usb.
This is going to be great, I don't know much about circuits but I can see how much thought and effort is going in to this. I can't wait.
Grant
I will use a screw terminal block, for several reasons. (1) the screw terminal block tightly holds the wires you screw into it (2) say we find the 20-pin connector, then all we have to do is crimp plug and connect cables to screw terminals (3) in case toyota decides to change the plug, we can just change the plug and not redesign the entire pcb (4) this allows the circuit to be used with other cars (5) the phoenix contact terminals i use are rather cheap.
hmm, ok sure, i forgot about the whole esd thing. I do want to make sure that the circuit works in multiple environments and is durable. I guess I should put in the cap as well, or are the diodes sufficient for esd protection?
The purpose of the cap is not for ESD protection. The cap forms the external reset circuit together with the pull-up resistor. You can leave out the cap and pull-up resitor if you want. This does however make the AVR more prone to unintentional resets as described in AV042 section 3.
But as was also already mnetioned, a rare unintentional reset would probably not be fatal to the system in any way. It would just restart the AVR and start reading the switches again. The PC should autodetect the USB HID "keybaord" connected again. You might not even notice a reset.
But on the other hand an extra resistor + cap is cheap and doesn't take up much board space, so it's probably not a deal breaker to include them. And a car can be a noisy enviroment.
If you use debugWire you should also note that this will not work if you have a cap attached to the reset pin as also mentioned in the documentation.
So it's up to you if you want to add them or not, but you really should add the diode for EDS protection under all circumstances. There's pros and cons for adding the cap + resistor as with many things. My guess is it will work fine without it.
And speaking of the reset pin, make sure not to turn off the reset pin by setting it as a normal input pin (do not set the RSTDISBL fuse). This would mean you can't use ISP programming anymore and would have to use high voltage programming to reset all the fuses before you can use ISP again. This is a common mistake many AVR programmers have made at some point.
Read this FAQ: http://support.atmel.no/bin/customer...wKbEntry&id=13
RSTDISBL fuse
Question
I programmed the RSTDISBL fuse and now I am not able to reprogram my AVR, what is wrong?
Answer
Please note that your AVR needs an external RESET source for the ISP interface to work.
When you programmed the RSTDISBL fuse you disabled the ISP interface at the same time.
To reenable the ISP interface you need to use High Voltage Programming to unprogram the RSTDISBL fuse.
To do this you need an STK500. The procedure and connections for High Voltage Programming is located in the AVR Studio Helpfiles:
AVR Tools on-line help --> STK 500 User's Guide --> Hardware description --> High Voltage Programming.
new schematic
new board
new render
I used some through-hole parts to make routing a little better and in place of vias. I mainly chose capacitors for that purpose.. and a 0 ohm through-hole resistor. about 1/2 the size of the original board now.
New render's looking great. I've finally got my car PC installed and I still find myself pressing the steering wheels controls but to no avail!
Is that AVR being programmed in Assembler or C?
I would aslo use some larger (possibly tantalum) capacitors for C1 and C2, especially fo C1. I know these are what's suggested as minimum values in the TI LDO datasheet, but the circuit is going to be connected to a long USB cable which makes a charge capacitor more important. I guess there's room for a larger C1 since you have made it leaded, but might be a good idea to chage it to round polarized capacitor so you can see what's + and what's - if you choose to use a tantal capacitor here.
Here's what TI writes about C1: http://focus.ti.com/lit/ds/symlink/tps71533.pdf
And the device must be said to possibly be more than just several inches away from the power source which is the PC through a long USB cable.A higher-value input capacitor may be necessary if large, fast-rise-time load transients are anticipated and
the device is located several inches from the power source.
Your have to move your USB connector closer to the edge of the PCB, otherwise there's no romme to put a plug in the connector.
You could use a leaded crystal and the place the two crystal capacitors directly below the crystal on the blue layer. The footprint of a leaded HC49 crystal is smaller than the footprint for an SMD HC49 crystal anyway becuse you don't need large pads to go outsie the crystal house. Doing this would make the traces for the high frequency crystal shorter and would take away board space on the left side. If you are not mounting the PCB by machine, then it doesn't matter if you have placed all components on the same side or not. Then you could take away some of the PCB on the left side which would also move the USB connector closer to the edge without moving it further away from AVR. You could also move J2 closer to the AVR.
You should remove D2 from your your circuit again, as it is a redundant diode and serves no purpose (see AVR042 figure 3-1).
The reset pin already has a diode to GND internally. It's only the diode to VCC that's missing to allow high voltage programming.
Everything inside the "Microcontroller" box is inside the AVR. Only C, R and the diode to VCC is not inside the AVR.
You should also move D1 further away from the screw terminal if you want to be able to solder D1. By removing D2 you will have room to turn D1 90 degrees which will solve this problem.
You can also safely remove either C5 or C6 and route the two digital VCC pins pin 4 and pin 6 together under the IC. One cap here is plenty, no reason to use two caps here. The leaded part doesn't even have two VCC pins, it's just because the SMD package has 32 pins instead of 28 pins, so they have to use the remaning pins for something. They might even be connected directly to each other internally. You can try to test it with an ohmmeter.
After looking at your PCB layout I don't see any purpose of the 0 ohm through hole resistor R6. Instead just place a single via where R6 is placed to move the red trace from the AVR to the blue trace of the ISP connector. No reason to add two vias (holes) and a resistor when a single via is plenty.
You have also routed pin 2 of the ISP connector to the wrong place, this should have been routed to VCC (3.3V) instead of the USB supply voltage as you have done. You don't have to worry about damaging the LDO with reverse current while you power VCC (3.3V) directly from the ISP with the USB connector disconneted.
Here's what the LDO datasheet says about this: http://focus.ti.com/lit/ds/symlink/tps71533.pdf
That said, have you considered turning the ISP connetor 180 degrees? Looks like this will offer better rounting.Regulator Protection
The TPS715xx PMOS-pass transistor has a built-in back diode that conducts reverse current when the input
voltage drops below the output voltage (e.g., during power-down).
Looking at your PCB layout and 3D rendering I can see the SMD caps and resitors are two different sizes. Maybe you should change them to all be either 0603 or 0805.
Have you considered designing the PCB to fit a small standard plastic case?
Looks like the schematic is from CadSoft Eagle?
Can Eagle also make 3D renderigns now, didn't know that? Looks like the 3D renderings from Altium Designer.
Ok, I'll take D2 out. AVR041/042 says to use a cap for each pair of vcc pins, so I just followed the appsheet =/. I chose to use a 0 ohm resistor instead of a via because I felt the resistor would be more durable than a single via (what would i put in a single via)...
I think the caps are 0402, I'll change the resistors. I was told to always route VCC from the isp to Vin of the regulator to avoid problems, but I can see the problem with usb (though, it shouldn't be plugged in to the isp and usb at the same time). I will take your routing suggestions... and there is a pov-ray plugin for eagle that i used to generate the render.
also, about the screw terminals and D1, wouldn't a solder D1 first, then the screw terminals? the only thing about placing smd components on the reverse side is that double sided silkscreen is extra.
Why would you put anything inside a via?? Via holes are often so small that it's not even possible to put anything inside them. When proferssional 2 layer PCBs are made, they use electroplating which will add the connection between top and bottom layers. Or even the midlle layers if you use more than 2 layers.
So you don't have to put anything inside vias, the connection is already made when the PCB is manufactured.
It's the same thing with the bigger component holes on 2 or more sided PCBs, here theres also already a connection made when the PCB is manufactureedd.
So there's really no reason to use leaded componets just to add a connetion between two layers.
Of course if you do homemade 2 layer PCBs the connetions in the vias might have to be made manually if you don't use electroplating as professional PCB manufacturers does.
Here's a description of how PCB vias work: http://en.wikipedia.org/wiki/Via_(electronics)
If you look inside the holes at the PCB you had made earlier you can see there's already a silver colored eletrical connetion inside all holes between top and bottom layer before any components are inserted:
You should never solder pads that borders directly up to a house of another component. Especially a large mechanical component like a terminal block. The mechanical stress from the screw terminal can soon ruin the solder joint at the pad it touches. At the 3D render it lookes like there's some room, but at the PCB layout there's no room at all. Depends on the type of terminal used.
But when you remove one diode, there's plenty of room to rotate the other 90 degreed (I wrote 45 degreees before but meant 90 degrees).
And you don't really need a silkscreen at the bottom for just two identical capacitors for the X-tal. It's esy enoguh to figure out where they go without a second silscreen at the bottom.
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