Serial decode depends upon trigger?

Forum for discussing PicoScope version 6 (non-automotive version)
Post Reply
Bmccand
Newbie
Posts: 0
Joined: Thu Aug 26, 2021 12:54 pm

Serial decode depends upon trigger?

Post by Bmccand »

Hello,
I'm a new user of a 2205A. I'm debugging a 9600 baud serial problem. Chan A set to input of a multiplexer box, chan B set to the output. The serial decode seems to work only for the channel which is the trigger. I've tried changing offset and hysteresis, no help. I'm attaching two psdata files. I think the trigger was A or B as noted in the file name, but View:properties doesn't show the trigger source.
The correct serial packet starts with "$PMRR"
I'm probably missing some setting here, so please advise.
Bill Mc
Attachments
B serial OK 210824-3.psdata
(114.47 KiB) Downloaded 267 times
B serial Bad 210824.psdata
(117.31 KiB) Downloaded 577 times

Gerry
PICO STAFF
PICO STAFF
Posts: 1034
Joined: Mon Aug 11, 2014 11:14 am

Re: Serial decode depends upon trigger?

Post by Gerry »

Hi Bmccand,

I apologise for the long delay in answering, I have been tied up with other work, so I haven't been able to answer forum posts for quite a while.

If you haven't resolved this yet, I have created a screen-shot of the measurement of the Baud Rate you are actually using (see below) i.e. 8.1k Baud.
Incorrect baud rate.png
With a stop bit, start bit, and 8 data bits, along with a bit width of 123.5us, the 8th data bit starts 9 * 123.5 = 1.1115 ms after the beginning of the start bit, the 7th data bit starts 8 * 123.5 = 988 ms after the beginning of the start bit, and the 6th data bit starts 7 * 123.5 = 864.5 ms after the beginning of the start bit. Assuming a fixed baud rate of 96k baud, the serial decoding is looking for the middle of the 8th data bit 8.5 * 1.0467ms = 8.854 ms after the beginning of the start bit (which is actually before the start of the 6th data bit).

So, because the Baud rate error is quite large, what could be happening is that the Serial decoder is having to perform a Baud hunt (adjust the clock to match your Baud rate of 8.1k Baud), which is why it only finds the correct value for a bit, 10 bits after the start of the capture where the Channel B decode is correct, and 12 bits after the start of the capture where the Channel A decode is correct.

The issue could be related to the Trigger, while your Baud Rate is incorrect (e.g. it could be not Baud hunting on the Trigger Channel). What happens if you use an Auto Trigger?

Regards,

Gerry
Gerry
Technical Specialist

Bmccand
Newbie
Posts: 0
Joined: Thu Aug 26, 2021 12:54 pm

Re: Serial decode depends upon trigger?

Post by Bmccand »

Gerry, thanks for the analysis. I'll have to re-connect the fixture to double check the hardware. I'll look at the snaps again to verify the technique. I just looked at amplitude before.
The problem arose with a GPS talking through a multiplexer to a radio. I tried direct wiring the GPS to radio and the problem still occurs. Now to scrutinize the baud rate.
Bill Mc.

Post Reply