GPIO pullups and direction

[JohnBrown]

If I write a zero to the output register, and enable the pullup on the same pin, can I switch between driven low output and input with pullup simply by settting/clearing the DIR bit? Will the pullup still be in circuit when the output is enabled? I have studied the datasheet and the associated schematic, but this is still not clear to me.

[Susheel Nuguru]

I have tried the both ,

NRF_GPIO->DIRSET |= (1UL << LED_1);
NRF_GPIO->PIN_CNF[LED_1] |=  GPIO_PIN_CNF_PULL_Pullup << GPIO_PIN_CNF_PULL_Pos;	

NRF_GPIO->DIRSET &= ~(1UL << LED_1);

---

NRF_GPIO->DIRSET &= ~(1UL << LED_1);
NRF_GPIO->PIN_CNF[LED_1] |=  GPIO_PIN_CNF_PULL_Pullup << GPIO_PIN_CNF_PULL_Pos;	

NRF_GPIO->DIRSET |= (1UL << LED_1);
while(1);

and the pullup wasn't there. So i am concluding that it will be reset to no_pull everytime we change pin direction

[JohnBrown]

Thanks for the answer, but I have to say that sounds like a crazy system to me! Some things about the nRF51822 are great, others, like this and the compromised PWM capability, are not so great.

[Susheel Nuguru]

I think this came as a side effect of being able to set direction either from DIR register or in PIN_CNF register. i will bring this to notice of right people.

[JohnBrown]

I think you are using the wrong register. It should be DIR, rather than DIRSET. I just wrote the following snippet of code:

NRF_GPIO->DIR &= ~(1UL << BUTTON_NORTH);		// Set as ip
NRF_GPIO->PIN_CNF[BUTTON_NORTH] |= GPIO_PIN_CNF_PULL_Pullup << GPIO_PIN_CNF_PULL_Pos; // Set pullup

SEGGER_RTT_printf(0, "PIN_CNF = %08x\n\r", NRF_GPIO->PIN_CNF[BUTTON_NORTH]);
NRF_GPIO->DIR |= (1UL << BUTTON_NORTH);		// Set as op
SEGGER_RTT_printf(0, "PIN_CNF = %08x\n\r", NRF_GPIO->PIN_CNF[BUTTON_NORTH]);
NRF_GPIO->DIR &= ~(1UL << BUTTON_NORTH);		// Set as ip
SEGGER_RTT_printf(0, "PIN_CNF = %08x\n\r", NRF_GPIO->PIN_CNF[BUTTON_NORTH]);

and the output on the Segger RTT shows the PIN_CNF rgister to be: 0x0000000e 0x0000000f 0x0000000e

So I believe the pullup does persist if you use the DIR register to chage direction.

I will re-write this to use the DIRSET and DIRCLR registers...

while(1);

[JohnBrown]

NRF_GPIO->DIRCLR = (1UL << BUTTON_NORTH); // Set as ip NRF_GPIO->PIN_CNF[BUTTON_NORTH] |= GPIO_PIN_CNF_PULL_Pullup << GPIO_PIN_CNF_PULL_Pos; // Set pullup

SEGGER_RTT_printf(0, "PIN_CNF = %08x\n\r", NRF_GPIO->PIN_CNF[BUTTON_NORTH]);
NRF_GPIO->DIRSET = (1UL << BUTTON_NORTH);		// Set as op
SEGGER_RTT_printf(0, "PIN_CNF = %08x\n\r", NRF_GPIO->PIN_CNF[BUTTON_NORTH]);
NRF_GPIO->DIRCLR = (1UL << BUTTON_NORTH);		// Set as ip
SEGGER_RTT_printf(0, "PIN_CNF = %08x\n\r", NRF_GPIO->PIN_CNF[BUTTON_NORTH]);

while(1);

0> PIN_CNF = 0000000E 0>
PIN_CNF = 0000000F 0>
PIN_CNF = 0000000E 0>

[JohnBrown]

Sorry, the code tages got screwed up there! But as you can see, if you use DIRSET and DIRCLR correctly, the pullup does seem to persist.

John