broadcom - Pin.c setting of JTAG pins -- should it have pull-down?

[jerryneedell]

When the Pins are reset in
https://github.com/adafruit/circuitpython/blob/main/ports/broadcom/common-hal/microcontroller/Pin.c#L53
for the JTAG pins 22-27, the Pins it set to ALT4 then it returns without setting a PullDOWN.
Looikng at the Datasheet, these pins are listed as having pull-downs.
section 5.3 https://datasheets.raspberrypi.com/bcm2711/bcm2711-peripherals.pdf

Sould the be set here?

It seem sot be working as it and it and I f set them as Pull-down it still works. I'm just curious if they should be set .

here is the change I made to test it.

diff --git a/ports/broadcom/common-hal/microcontroller/Pin.c b/ports/broadcom/common-hal/microcontroller/Pin.c
index 1cf86f2b3..f3355a7be 100644
--- a/ports/broadcom/common-hal/microcontroller/Pin.c
+++ b/ports/broadcom/common-hal/microcontroller/Pin.c
@@ -48,14 +48,15 @@ void reset_pin_number(uint8_t pin_number) {
     pin_in_use[pin_number] = false;
     never_reset_pin[pin_number] = false;
     // Reset JTAG pins back to JTAG.
+    BP_PULL_Enum pull = BP_PULL_NONE;
     if (22 <= pin_number && pin_number <= 27) {
         gpio_set_function(pin_number, GPIO_FUNCTION_ALT4);
+        pull = BP_PULL_DOWN;
         return;
     } else {
         gpio_set_function(pin_number, GPIO_FUNCTION_INPUT);
     }
     // Set the pull to match the datasheet.
-    BP_PULL_Enum pull = BP_PULL_NONE;
     if (pin_number < 9 ||
         (33 < pin_number && pin_number < 37) ||
         pin_number > 45) {

[tannewt]

I looked at the current state of pins prior to the first reset and they didn't all match the datasheet. So, I'd go by what works rather than what the datasheet says.

[jerryneedell]

Sounds good. Thanks.

[jerryneedell]

Actually, one more question...
If you set it to ALT4 and return but don't explicitly set the Pull (up/down) then I guess it just stays as it was. Is that OK or do you want to be explicitly setting it to something?

or is it that pull only has meaning if it is set as in input? so it it irrelevant for ALT4
the data sheet does say "The Alternate function table also has the pull state which is applied after a power down."

Interesting difference in the BCM2711 data sheet and the BCM2835 datasheeet
BCM2711

GPIO_PUP_PDN_CNTRL_REG0 Register
Description
The GPIO Pull-up / Pull-down Registers control the actuation of the internal pull-up/down resistors. Reading these
registers gives the current pull-state.
The Alternate function table also has the pull state which is applied after a power down.

BCM2835

GPIO Pull-up/down Register (GPPUD)
SYNOPSIS
The GPIO Pull-up/down Register controls the actuation of the internal pull-up/down
control line to ALL the GPIO pins. This register must be used in conjunction with the 2
GPPUDCLKn registers.
Note that it is not possible to read back the current Pull-up/down settings and so it is the
users’ responsibility to ‘remember’ which pull-up/downs are active. The reason for this is
that GPIO pull-ups are maintained even in power-down mode when the core is off, when
all register contents is lost.
The Alternate function table also has the pull state which is applied after a power down.

[tannewt]

Ya, we should probably reset it to the state we want. That's good to know about reading pull back. I think I'm doing that now.

[jerryneedell]

Ya, we should probably reset it to the state we want. That's good to know about reading pull back. I think I'm doing that now.

Would you like me to submit a PR for the change I made above to set them to pull-down or since you are likely making other changes do you want to do that yourself?

[tannewt]

Go ahead @jerryneedell. I'm working on Zero support.

[tannewt]

This is done in #5798. Thanks @jerryneedell