SWM on LPC845: connecting to hardware with clocks disabled

[loudpotato]

Hi. I am looking into configuring pins on LPC845, SWM documentation says:

Before activating a peripheral or enabling its interrupt, use the switch matrix to connect the
peripheral to external pins.

Let's say I assign U0_TXD to PIO0_0 but leave UART0 without clocks in SYSAHBCLKCTRL0. What state of the PIO0_0 will be (driven high, hi-z, something else)?

[gusarambula]

Hello Loudpotato,

The PIO0_0 state out of reset would be Input, internal pull-up enabled.

Once you have configured it as U0_TXD it would be an output, as this signal is the transmitter output. But the state would depend on the UART module, and I couldn’t find a known default state when the UART module has its clock disabled.

My apologies for the inconvenience.

Regards,
Gustavo

[loudpotato]

Hi Gustavo, thank you for looking into this.

I looked into the USART section of UM11029 and it says

Configure the USARTs for receiving and transmitting data:
• In the SYSAHBCLKCTRL register, set bit 14 to 16, bit 30, and 31 (Table 146) to
enable the clock to the register interface.

...
• Configure the USART pin functions through the switch matrix.

Which makes sense to me. I realize UM11029 is a large document, probably it was just a mistake in the SWM section.