Seeking for some assistance to wire components

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Hey!

 

So i pretty much got the PCB designed, but stumpled upon couple of things i'm not so sure about and would like to get some input about.

 

So first one, The OE pin of TXB0102 Should it be driven by levels of VCCA or VCCB? or does it even matter, the best for me would be if i'm able to drive it via VCCB.

 

 

second one, this is in series of stupid questions to be honest, How do i calculate RC values? Or Could i just go and use only pull-up on the reset pin, then reset via SPI since its suggested that only HW or SW reset is needed.

third one, Does the connection look allright? like usually one would reverse TXD/RXD for uart, but ii dont think that applies here with CAN? 

Still need to figure if i'm able to somehow run the MCP2561 standby pin via MCP2515, after all its coming to a car and run by constant power line.

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Hi Joni -

 

The TXB0102 spec sheet Pin Function Table says that OE is referenced to Vcca. Could not find any reference to design considerations for the series resistor for OE. I would try a couple of Kohm (maybe 1.0K to 2.2K) but prepared to install a jumper.

 

On the MCP2515, it says  that R and C should be chosen to hold the reset low for at least 2us after power-up. I would try 10K and 0.01uf which has a time constant of 0.1ms = 100us. That is the time it takes C to charge to 63% of its final value. That should keep it as logic low long enough.

 

Jim 

 

 

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Hey, Jim!

 

I did read the datasheet couple of times again for the TXB0102 and could not find anything about the series resistor either, and to make it worse while searching i found someone claiming that the chip does not play nicely with uart in 3v3(A) and 5.0v(B) setup which is exactly what it was for in my design, i thought it was supposed to work in that, but i don't want to risk it.

 

So yeah, it might be that i look for other level translator between my BLE module and MCU.

 

E: i'm also still missing some small footprint relay which would work with 5v if someone got any suggestions. (switching Max 14.4v and 500mA current)

 

 

 

Last Edited: Mon. Nov 19, 2018 - 02:26 PM
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JoniS wrote:
... after all its coming to a car and run by constant power line.
A car doesn't have constant power.

After engine start, the battery as current sink has priority for the current source (alternator); after the starter battery's SoC reaches a minimum, then other loads receive more current.

The battery's initial charging duration varies by initial SoC and air temperature (might be a few minutes near 0C; assumes a healthy alternator)

More cars (HEV) have 24V and 48V systems so obviously there's current limiting; some automotive MCU DC-DC is from a buck-boost such that the MCU is completely operational.

The car's body computer may, or may not, control secondary loads; primary loads may be controlled by the ECU since the ECU has to have sufficient power.

Medium automotive (medium trucks, semi-tractors, buses) is 24V with 12V auxiliary.

 

"Dare to be naïve." - Buckminster Fuller

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gchapman wrote:
A car doesn't have constant power.

 

What i did intent to say is that components on the PCB will be directly powered from battery with own power line, so there will be power even when the key is removed, thus all the power safe modes should be utilised where possible.

 

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JoniS wrote:
I did read the datasheet couple of times again for the TXB0102 and could not find anything about the series resistor either, ...
fyi, Arduino Uno WiFi rev2 has TXS0102 for bi-directional TWI and UPDI.

I don't remember the difference between TXB and TXS.

https://content.arduino.cc/assets/UNOWiFiREV2V4.0_sch.zip (page 6 for the 2-bit level translator)

via ARDUINO UNO WiFi REV2 (Documentation tab)

 

edit: typo

 

"Dare to be naïve." - Buckminster Fuller

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There should be a switched battery power terminal "some where" (car's computers handle low battery)

Otherwise, does your design have a low battery shutdown function?  (preferably completely in hardware)

 

"Dare to be naïve." - Buckminster Fuller

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if i remenber correctly from my searching the TXS model is happy with open drain and Push-Pull logic, where as TXB only with Push-Pull.

 

So i could just use resistor and Cap in the OE pin, and keep the chip always enabled.

 

There should be a switched battery power terminal "some where" (car's computers handle low battery)

Otherwise, does your design have a low battery shutdown function?  (preferably completely in hardware)

Excellent question, and the answer is no. But now when you did mention it, i think i must implement it. Honestly that did not even come to my mind since i'm replacing the old battery soon and more than likely add another one to trunk same time. 

 

There should be a switched battery power terminal "some where" (car's computers handle low battery)

There is ACC line behind the headunit if i remenber correctly which i intend to use for other stuff, so it will be connected to the PCB. (Almost everything is controlled by CANBUS in my car, even the headunit power on/off so its kind of funny they did still route switched 12v behind it in first place, but well thats good for me)

Last Edited: Mon. Nov 19, 2018 - 03:23 PM
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Otherwise, does your design have a low battery shutdown function?  (preferably completely in hardware)

 

Gotta laugh, (or cry...), as I overlooked that a few years ago for the project shown below.

It is/was a GPS & Accelerometer vehicle tracker, that auto uploaded the data to a PC via Bluetooth whenever the vehicle was on station.

(Emergency vehicle driver surveillance, speed, "jerkiness" of driving, etc.)

 

I ran the prototype, shown, in my car and if the car sat for a few days, unused, it drained the car battery down to the point where the vehicle wouldn't start.

 

Lessons learned:

Some designs are better than others

That's why we call it a prototype

 

JC

 

 

  

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DocJC wrote:
I ran the prototype, shown, ...
It's good looking for a first prototype; a cross between a bench test (proof-of-concept) and a second prototype.

Lots of PCB fabs have the 2-layer no-mask deals some of which are overnight fab and ship.

DocJC wrote:
... it drained the car battery down to the point where the vehicle wouldn't start.
A TL431 driving an IPS or a PNP can consume 1mA which might be acceptable; improved TL431 (ATL431, AS431I, etc) are an order of magnitude less current.

New from TI at 7microA typ Iq : TLV6713 36-V Comparator with Integrated Reference | TI.com

 

"Dare to be naïve." - Buckminster Fuller

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 it drained the car battery down to the point where the vehicle wouldn't start.

In other words,  it also included an advanced anti-theft feature. 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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avrcandies wrote:

 it drained the car battery down to the point where the vehicle wouldn't start.

In other words,  it also included an advanced anti-theft feature. 

If it was product from Apple then that for sure was a feature.

But anyway, I'm going to figure out something to make sure that won't happen.