Teensy 4.1 & UART

D

dwiea

Member
So I have been working on a PCB design for a while using the Teensy 4.1 but I am not sure if I have overlooked something. I have a requirement for 6 UART and to use the extra soldered on QSPI PSRAM & Flash installed along side the SD Card.

Just been looking at the reference sheet and it seems like RX8 is connected to one of the QSPI memory chips and TX5 & RX5 is attached to the SD card (although in the diagram these boxes are greyed out). Does this mean I cannot use these UART pins on the board if I am using the SD Card / extra memory chips?

https://www.pjrc.com/teensy/card11b_rev4_web.pdf
 
All the uarts should be available on standard pins.

PortTxRx
Serial 110
Serial 287
Serial 31413
Serial 41716
Serial 52021
Serial 62425
Serial 72928
Serial 83534

The greyed out pins you are looking at are alternative pins that can be used for the same function but aren't the default.
 
AndyA said:
All the uarts should be available on standard pins.

PortTxRx
Serial 110
Serial 287
Serial 31413
Serial 41716
Serial 52021
Serial 62425
Serial 72928
Serial 83534

The greyed out pins you are looking at are alternative pins that can be used for the same function but aren't the default.
Click to expand...
So after a quick look at the pinout cards I could in theory use all of the serial ports available with extra memory chips and SD card?
 
Oh and I see setTX and setRX methods in the documentation. Does that mean that I can choose pins that I am not using on a different side of the board that are better for routing tracks on my PCB i.e. move RX1 and TX1 to A17 & A16?
 
dwiea said:
Oh and I see setTX and setRX methods in the documentation. Does that mean that I can choose pins that I am not using on a different side of the board that are better for routing tracks on my PCB i.e. move RX1 and TX1 to A17 & A16?
Click to expand...

No. Pins aren't completely flexible.

There are normally more possible IO features than there are IO pins on the processor which means that some pins have to have more than one possible function. But then you hit the issue of what to do if you want both of the functions on a given pin.

Ideally you'd have a huge switch matrix so that any internal function can be mapped to any IO pin. But that would take up a huge amount of silicon and add extra delays making things both slower and more expensive.
The way most manufacturers get around this is by offering a compromise. Any internal function can be mapped to any one of 3 or 4 pins with pins shared between functions such that almost any combination of the internals can be used (within the limit of the total number of pins) if you set the pin mapping correctly.

On the Teensy pinout diagrams the functions in black are the pins that are used as default for that peripheral if you don't set the pin. The ones in grey are the alternative pins you can optionally set that peripheral to use. There are actually a few extra alternatives for some of the pins that aren't listed for simplicity on the standard diagram.
https://forum.pjrc.com/index.php?threads/teensy-4-1-extra-pins.61294/#post-242695 gives a more complete diagram.
 
AndyA said:
No. Pins aren't completely flexible.

There are normally more possible IO features than there are IO pins on the processor which means that some pins have to have more than one possible function. But then you hit the issue of what to do if you want both of the functions on a given pin.

Ideally you'd have a huge switch matrix so that any internal function can be mapped to any IO pin. But that would take up a huge amount of silicon and add extra delays making things both slower and more expensive.
The way most manufacturers get around this is by offering a compromise. Any internal function can be mapped to any one of 3 or 4 pins with pins shared between functions such that almost any combination of the internals can be used (within the limit of the total number of pins) if you set the pin mapping correctly.

On the Teensy pinout diagrams the functions in black are the pins that are used as default for that peripheral if you don't set the pin. The ones in grey are the alternative pins you can optionally set that peripheral to use. There are actually a few extra alternatives for some of the pins that aren't listed for simplicity on the standard diagram.
https://forum.pjrc.com/index.php?threads/teensy-4-1-extra-pins.61294/#post-242695 gives a more complete diagram.
Click to expand...
Ah, I thought the whole XBAR thing was something to do with remapping serial pins. If it isn't then I'm not sure what it is about :)
 
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