topic Re: Serial Voltage levels in 8-bit Microcontrollers

Serial Voltage levels

karthick1987 — Thu, 19 Aug 2010 12:24:29 GMT

Hi guys,

I am having trouble with the serial voltage levels. I hooked up the CRO to the Tx pin and found that the low-level goes to about 1.8V and the high goes all the way to 3.3V, what do you think is the problem with the serial ?

I checked the ground and it seems to go to 0V

Cheers

Karthick

Re: Serial Voltage levels

karthick1987 — Thu, 19 Aug 2010 14:19:57 GMT

The settings that I am using on the GT16A are

loop - disabled

TxD pin direction - ouput

Data format - 8 bits

Parity - none

Send break - disabled

baud rate - 9615.385 bps

Transmitter - enabled

But the output low I am getting is about 1.7V ! Is this a hardware fault? Could this mean that the chip is damaged?

Because the TPM and ADC and everything else works fine. I am a little confused.

Someone please help me out.

Cheers

Karthick

Re: Serial Voltage levels

peg — Thu, 19 Aug 2010 14:29:30 GMT

Hello Karthick,

Set the direction of both SCI pins to input.

It should not matter as enabling the SCI should override this but I have seen strange similar problems.

When I went to test it again latter it seemed to work OK and I could not duplicate the problem so took it no further.

Please come back and report here if doing the above fixes your problem or not.

Re: Serial Voltage levels

karthick1987 — Thu, 19 Aug 2010 22:06:32 GMT

This problem is weird but what you said worked!

Thank you very much Peg, Just out of curiosity why does this happen?

Re: Serial Voltage levels

peg — Fri, 20 Aug 2010 05:15:35 GMT

Hi,

It seems that under some condition you can have the general purpose i/o and the SCI controlling the pin at the same time. So generally the GPIO is stuck at its initialised level and the SCI TDO pin tries its best to pull it the other way. I have had this happen twice with the GT16A. But when I experiment later to try to see what is going on I can't get the problem to occur again. Once you set it to input I have never seen any problem.

I was intending to bit bang a strange protocol first in order to reconfigure an attached piece of equipment to a more normal protocol that the SCI could handle. That is why I was deliberately setting the TDO pin to output.

Re: Serial Voltage levels

karthick1987 — Wed, 25 Aug 2010 20:13:08 GMT

Hi guys,

Just noticed that the problem was not fixed after following what peg said. It is pretty erratic, very very weird.

Cheers

Re: Serial Voltage levels

bigmac — Thu, 26 Aug 2010 13:54:18 GMT

Hello K,

What do you have externally connected to the TxD pin? Try disconnecting the external load and see if the problem persists.

Regards,

Mac