3.3V to 5V communication

lpcware · ‎06-15-2016

Content originally posted in LPCWare by Hubert Wu on Tue Apr 27 19:02:38 MST 2010
Hi,

I want to use SPI and UART of LPC1111 to communicaiton with 5V system. As we know the I/O has 5V tolerant. Should I add any components with that? If I add pull high resistor to 5V only. Is that OK?

lpcware · ‎06-15-2016

Content originally posted in LPCWare by larryvc on Tue Apr 19 13:31:30 MST 2011

Quote: Rob65
Yes, you are wrong - No, you are not :confused:

I understand fully what you described.:confused::)

Quote: Rob65
I have always added extra drivers (just a simple mosfet) to create 5V (or higher) outputs.

Very good practice.

Thanks,
Larry

lpcware · ‎06-15-2016

Content originally posted in LPCWare by Rob65 on Tue Apr 19 12:58:41 MST 2011

Quote: larryvc
e it does not appear that the pullups would be available when configured as an output. See figure 32. If I am wrong please let me know.
Larry

Yes, you are wrong - No, you are not :confused:

Indeed, the weak pull-ups as given in figure 32 are only used when a pin is configured as an input pin. I was talking about a pull-up towards the 5V supply to allow 5V devices (inputs) to be attached to the 3.3V lpc1xxx. An external pull-up would be needed in order to generate a logical 5V signal since the lpc1xxx itself will only give 3.3V on an output pin.

I have always added extra drivers (just a simple mosfet) to create 5V (or higher) outputs. Inputs (up to 5V) are OK as long as you are sure 5V is the absolute maximum and that the 5V is only applied after Vdd is applied.

lpcware · ‎06-15-2016

Content originally posted in LPCWare by larryvc on Tue Apr 19 12:08:03 MST 2011

Quote: Rob65
I cannot determine if it is allowed to have a pullup to 5V when using a pin as an output pin.

To me it does not appear that the pullups would be available when configured as an output. See figure 32. If I am wrong please let me know.

Larry

lpcware · ‎06-15-2016

Content originally posted in LPCWare by Rob65 on Tue Apr 19 11:42:13 MST 2011

Quote: cyberstudio
I read the LPC111x User Manual and found no qualifications on 5V tolerance other than the fact that if you configure the pin as analog than you lose 5V tolerance

Look at the datasheet.
The User Manual tells you how to use the chip and it's peripherals. The datasheet mentions all the (electrical) conditions that apply.

Table 3, 4 and 5 in the datasheet of the lpc1114 (link) has a not for almost every pin stating it is 5V tolerant - a separate note states that ADC pins are not 5V tolerant when configured as analog input.
Table 7 in that same datasheet shows the exact values that apply.
Although the notes below the tables state "5 V tolerant pad providing digital I/O functions" I cannot determine if it is allowed to have a pullup to 5V when using a pin as an output pin. looking at the rest of the datasheet it looks like the output uses a totem pole output. But then they do mention an Ipd parameter in table 7 with a constraint of "Vi = 5V" so from there it seems to be OK.

Rob

lpcware · ‎06-15-2016

Content originally posted in LPCWare by cyberstudio on Tue Apr 19 10:45:03 MST 2011
I read the LPC111x User Manual and found no qualifications on 5V tolerance other than the fact that if you configure the pin as analog than you lose 5V tolerance. So I assume there are no perils just pulling up outputs to 5V. (But I personally always use a 74HCT powered at 5V to do 3.3V to 5V level translation - you won't need to be negatively impacted by the waveform cleanliness versus power consumption trade-off incurred by a pull-up resistor.) Certain other brands of microcontrollers have additional restrictions on 5V tolerance, such as the pin shall not be configured as output, you shall not use pull-ups, or other restrictions.

In particular, I would like to be absolutely certain that the hysteresis input of a LPC111x is 5V tolerant, since I want to use it that way. If anyone knows for sure, let me know. Thanks!