DK-TM4C129X UART start bit

johan hendriks

Other Parts Discussed in Thread: TM4C129XNCZAD

Hello,

recently ported an application to DK-TM4C129X which contains the TM4C129XNCZAD. I noticed some problems in serial communication with a device hooked up to the RX/TX on the board. The device would not respond as I would expect it to do (it only responds to a single command, so I decided to debug the TX line towards the device).

My UART settings are:

- 9600bps

- 8 data bits

- 1 stop bit

- no parity

To debug the TX output I contiuously output 0xAD (10101101). Attached image is the result on my scope. As can be seen the start bit is a '1'. Shouldn't this be a '0'?

over 11 years ago

0 Petrei over 11 years ago

Guru 26105 points

Hi,

May I ask you how did you initialized the system clock? i.e. porting is from TM4C123x? if yes, then look at my replay in this thread:

http://e2e.ti.com/support/microcontrollers/tiva_arm/f/908/t/353246.aspx

Petrei

0 johan hendriks over 11 years ago in reply to Petrei

Intellectual 325 points

Thanks for your reply. My system clock is configured as:

    clk = SysCtlClockFreqSet(
           (SYSCTL_XTAL_25MHZ | SYSCTL_OSC_MAIN | SYSCTL_USE_PLL
                   | SYSCTL_CFG_VCO_480), 120000000);

I ported my application from LM3s9b92.

0 Petrei over 11 years ago in reply to johan hendriks

Guru 26105 points

Hi,

You synchronized your scope on leading edge, please reverse to the falling edge.

Petrei

0 johan hendriks over 11 years ago in reply to Petrei

Intellectual 325 points

Hi Petrei,

here is falling edge capture.

0 cb1 over 11 years ago in reply to Petrei

Guru 47900 points

Petrei just (stole) my reply. (re: Sync Edge - your scope)

That said - a simple Led or DMM reading of UART_TX when "idle" should confirm the, "resting level" of TX. (indeed it should rise & stay high - so that the eventual arrival of, "Start Bit" may be easily detected.)

[edit] just saw your new (negative scope trig). Believe that you need to insert/enable some delay between UART character outputs - so that the return to UART idle (level) may be more clearly noted...

0 johan hendriks over 11 years ago in reply to cb1

Intellectual 325 points

A measurement on TX in idle indicates high (3.3V).

0 cb1 over 11 years ago in reply to johan hendriks

Guru 47900 points

johan hendriks said:
A measurement on TX in idle indicates high (3.3V).

Good that. (normal/customary)

Suggest that you now display 2 such character transmissions - with (ideally) a recognizable 150uS delay between each character. (i.e. @ 9600 baud - bit time is ~100uS - thus 150uS serves to better "mark" the UART idle time/state)

0 Amit Ashara over 11 years ago in reply to johan hendriks

TI__Guru**** 244440 points

Hello Johan,

I inspected the waveform plot and it shows a START BIT followed by the pattern LSB to MSB and then 1 Stop Bit. It then repeats.

I think you are running the code in a continuous loop. So just send one character and capture it.

Regards

Amit

0 johan hendriks over 11 years ago in reply to Amit Ashara

Intellectual 325 points

Attached is a capture of a single 0xAD.

oh...did I have LSB/MSB sideways...if that is the case than I'm confused as to why the device is not responding. It should be responding to the command I'm sending at 9600bps.

0 Amit Ashara over 11 years ago in reply to johan hendriks

TI__Guru**** 244440 points

Hello Johan

It is LSB to MSB. Attached is the snapshot of the UART character frame from the data sheet UART section. Check the other device. Is it expecting LSB first or MSB first

Regards

Amit

0 johan hendriks over 11 years ago in reply to Amit Ashara

Intellectual 325 points

Just checked the lm3s9b92 datasheet and there is it lsb first as well, so from my cpu point of view it should be fine. Time to check out what is going on with the device as it did respond to the lm3s9b92 sending the commands. Thanks for your input!

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DK-TM4C129X UART start bit