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sbcshop / Pitalk_4G_HAT_Hardware

The PiTalk 4G HAT is a handy, low-power Raspberry Pi HAT that features multi-communication functionalities such as GSM, GPRS, TCP, etc. It allows your Pi to easily make phone calls, send messages, connect to wireless networks, and so on. It is the most convenient IoT HAT that works with all variants of Raspberry Pi (model A, B and Zero). The users
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Reset pin 18 incompatible with LetsTrust TPM Module #1

Open ncul777 opened 3 days ago

ncul777 commented 3 days ago

I was hoping to use following Module as well as the PiTalk 4G modem, however I noticed that neither work when both plugged in at the same time. Both work when connected to the Raspberry Pi individually. I ensured that the TPM module was connected to the right pins in both cases

https://thepihut.com/products/letstrust-tpm-for-raspberry-pi

After some investigation I found that pin 18 is being used for reset on both modules, however

When both modules connected essentially both are held in reset (voltage on Pin 18 is approx 1.5 V ) When only TPM module connected the voltage is 2.75V

Is there any solution and/or workaround (e.g. around TR1)?

I've also informed LetsTrust about the issue.

PaulKissinger commented 3 days ago

Hi @ncul777, Hi sbcshop-team,

In my Point of view: both the HAT an the LetsTrust-TPM module are not direct compatible together.

I've reviewed schematics from the HAT from the PI and also from the LetsTrust-TPM (LTT) ->

The RPi itself has a default pulldown resistor with 50k Ohms.

If you have a look on the Datasheet of the NPN transistor, you'll find the 4k7 and a 47k Ohm resistor "in line" connected from BMC.GPIO(24) to GND. Results in an additional PullDown with 51k7Ohms, worst case 29k6 Ohm [1].

From the LetsTrust-TPM module, you have an 10k Ohm pullup resistor to 3V3.

Rpulldown_std = (50k||51k7) -> 25k4 Ohms Rpulldown_min = (50k||29k6) -> 18k6 Ohms

Theoretical voltage on BMC.GPIO(24) => (3V3/(10k+18k6)) * 18k6 = 2,36 Volts

From my side i have no idea where the other 650mV disappears...

[1] https://fscdn.rohm.com/en/products/databook/datasheet/discrete/transistor/digital/dtc043zebtl-e.pdf

If you have only 1-2 devices, my recommendation would be: cut pin 18 from the HAT, connect LTT on the top as usual and rewire the LTT-Reset-line with a jumpercable from an unused GPIO.

Best Regards,

Paul