Hello. The nRF51822 Series Reference Manual says on page 38, "The SWDCLK pin
has an internal pull down resistor and the SWDIO/nRESET pin has an internal pull up resistor."
What are the values of these pull resistors? If SWDCLK has an internal pulldown, why is an external pulldown required in the hardware design?
Thanks very much -
The pullup and pulldown resistance values is specified in nRF51822 PS v2.0, table 56.
On the first hardware revision of nRF51 (see the nRF51 compatibility matrix to identify nRF51 revision), a pulldown resistor was needed on the SWDCLK pin. On nRF51 second and third revision the resistor is not needed as it is present internally on the nRF51. However, a few customers have applied also an external pulldown resistor to make the pulldown stronger. These few customers were experiencing some signal noise on their PCB, causing a spike on the SWDCLK line and putting the nRF51 therefore into debug mode, which results in high current consumption. We have tested 470ohm external resistor on the SWDCLK pin and that has proven to be good for most programmers/debuggers, they can still drive the SWDCLK line
Thank you. Table 56 is under the GPIO specs. Are you saying the same pull resistors are also used on the debug interface?
That is correct.