Re: On the TX signal, 5V tolerant I/Os and Pull-up resistors
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Hi Fred?
I don't agree... When the pin is an output, the processor sets it at either 0V or 3.3V. Provided its current handling capability isn't abused, it doesn't matter what voltage it is "pulled up" to. It could be 1,000V provided the resistor was big enough, nobody would care.?
A 5V tolerant IO pin can withstand being connected to a low impedance 5V source. There are several clamping diodes in series so they don't draw huge current if the IO pin is connected to +5V.?
Those five pull up resistors are, categorically, totally unnecessary and totally harmless. Or I'll eat my hat.?
By all means remove them if it makes you feel better. I removed them in Rev 2. But not because they worried me at all. Just because less components is always better, even if only slightly so. More ground plane, less cost. Etc.?
I'm operating my QDX here every day without issues, on 40m and 20m. I've switched to 80m and 30m too but for receive only because my antenna isn't designed for 30m or 80m.
73 Hans G0UPL http://qrp-labs.com
-------- Original message -------- From: Fred Spinner <fred.spinner@...> Date: Mon, Nov 8, 2021, 7:27 PM To: [email protected]Subject: Re: [QRPLabs] On the TX signal, 5V tolerant I/Os and Pull-up resistors Interesting.? I am reading that you should not have those pull-ups to 5V there at all if the pin is used as an output.? The inputs have clamping diodes to clamp the 5v input to 3.3v.? If anything having a 5v pull up will burn off current for no gain in the STM32 through one of those clamping diodes.??
It probably makes sense to remove those pull-ups if for anything else lower current draw.? I would normally agree that the risk of damage doing this is low due to the 10k value of pull-up, but there is at least some circumstantial evidence that you might be seeing TX and LPF switching problems now from what is being reported here.? ?Could it be this?? I honestly think it could be.?
So yeah, I will probably pull them on mine.? Again, couldn't hurt, might help.?
Fred W0FMS Hi Fred
The 5V tolerant IO and the pull-up resistors to?+5V are all a red herring.?
According to the BS170 datasheet, the device has a gate threshold voltage of typically 2.1V but it could be as low as 0.8V or as high as 3.0V. I was nervous about?3.3V logic perhaps not driving it at sufficient voltage to be properly "ON". In other words not much safety margin between the 3.3V operating voltage of the microcontroller - bearing in mind the output will?not quite be rail-to-rail - and the worst case gate threshold?of the MOSFET.
My plan therefore was to use 10K pull-up resistors to?+5V. Then when I wanted the MOSFETs "On", I would set the I/O pin to be an input; and the 10K resistor would pull the MOSFET gate to?+5V. When I wanted the MOSFET to be "Off", I would set the I/O pin to be an output and set its level low. This is why you see these 10K pull-up resistors at the gates of the Tx/Rx switch Q6, and the three band select switches, and Q2 (on the TX signal).?
HOWEVER... I was mistaken. Because the transistor I used is BSS123 and according to its datasheet, its gate threshold voltage is minimum 0.8V, typical 1.7V, and maximum 2V. Therefore there is a lot more head room than I had expected with a BS170. So in the end, I did NOT keep to my original plan. I simply have the TX, RX and Band control signals all active all the time as Outputs, and set to high (3.3V) or low (0V) by the firmware. In either case (high or low), the pull-up to?+5V is of no consequence. It is an 0.5mA annoyance when the pin is at 0V, and a 0.17mA annoyance when the pin is at 3.3V. Nothing else.?
The microcontroller is not operated out of its spec. There is no possibility of damage. The pull-up resistors are ignored by the current firmware. Irrelevant. Unnecessary.?
As a matter of fact, in Rev 2 of the PCB, I simply deleted those pull-up resistors. I am talking about R5, R10, R11, R12 and R14. They are not required.?
Fred if you want to remove these resistors please go ahead. It makes no difference...?
73 hans G0UPL
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