LPC546XX: SDIO and external pull-ups

[giusloq]

I noticed LPCXpresso54628 EVB mounts external pull-ups resistors on data, CLK and CMD lines. In the examples, during pin configurations, internal pull-ups are disabled.

Is is possible to avoid external pull-ups and activate internall pull-ups on CLK, CMD and data lines? This could save some space on PCB.

[ZhangJennie]

Hi giusloq 

Normally, the internally pull up is week pull up as below pic. We use transistor to simulate resistance as internal pull up. it's tens of KOhm.

ZhangJennie_0-1652172016874.png

According to LPC546xx UM, Ipu is 6-30uA when VDD < VI < 5 V

ZhangJennie_1-1652173230016.png

Weak pull-up is that when the output is high, the current that can be output is very small, and it is easy to be pulled low by other strong pull-up, so it is called weak pull-up. For example, the pull-up current is 30 uA. The pull-up capacity is determined by the size of the pull-up resistance. 

So I suggest you using external pull up as reference design suggested.

Thanks,

Jun Zhang