Packet Error Rate - INT_RDA Issue

MotoDan · Joined: 30 Dec 2011 Posts: 55

Hello all,

I'm having a little trouble getting my RDA interrupt to reliably receive packets which are being transmitted from my VB app running on a PC.

My compiler is PCH V4.129 and I'm using a PIC18F which is currently running at 64 MHz. I also have two timer interrupts - Timer0 interrupts at 1 kHz and Timer4 interrupts at 25 kHz. Neither timer interrupt is doing much more than setting a flag or changing an output pin.

My RDA interrupt is set to HIGH priority and is used to fill a small buffer with a 6 byte packet. The UART is running at 115,200 baud. The packet includes a 1 byte checksum which is used to validate the packets.

The main program looks for the receipt of a valid packet flag which is set in the RDA interrupt and responds with a similar packet when a particular address and command are received.

Everything works great - Except that my packet error rate is about 15-20%. I've tried reducing the baud rate as low as 9600 without any improvement in the PER. I've also tried turning off both timers without any improvement in the PER.

I feel like the RDA interrupt/buffering is the cause because my PER goes to 0% when I dedicate the code to looping on the packet reception rather than using the RDA interrupt.

I've looked at the RX/TX on the scope and have also verified the correct packets are being sent using the scope's serial decoder. I can see when the PIC fails to reply to the previous packet.

At this point I think I'm either missing a byte here or there or my buffer pointer is getting out of sync.

Any idea on how to proceed would be much appreciated.

MotoDan

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Post a short but compilable test program that we can look at. Post
everything we need, so we can drop it into MPLAB and test it.

newguy · Joined: 24 Jun 2004 Posts: 1909

Have you had a look at your chip's errata? The UARTs on most PICs seem to be the subject of a lot of errata.

Ttelmah · Joined: 11 Mar 2010 Posts: 19537

Agree with the comments above.

However I 'worry' about the comment about the 6byte packet.
First thing is you do realise that INT_RDA means _one_ byte is available. No more. Then what happens if the first byte of a second packet starts to arrive before the main code has processed the first?.
For a six byte receive like this, I'd be looking at something like:

MotoDan · Joined: 30 Dec 2011 Posts: 55

Thanks for everyone's replies and insights.

The 6 byte packet gets assembled 1 byte at a time via the RDA isr. The trick is to get the receive buffer pointer initialized before the start of each packet. In my case, the packets are commands which have a minimum of 100 ms between them. This gives me time to re-sync the receive buffer pointer before the next packet arrives. My problem with occasionally missing packets was due to this re-syncing. Originally I was going to go with a unique SOH/STX byte, but other data in the packet could keep that from working.

Two changes were made to fix the problem. First, I corrected a problem in my buffer pointer syncing. I also moved all packet parsing to the main loop. The RDA isr now only stuffs bytes into the packet and sets a packet ready flag. Main now only parses new, full packets.

The packet error rate is now 0.00%! There are actually 4 interrupts running. Two for the dual UARTs (115,200 and 4,800 baud) and two timers (1 kHz & 25 kHz). The PIC18F26K22 seems to be more than able to handle everything. I'm sure the 64 MHz clock speed is responsible for a big part of this.

Thanks again for the help. This forum is such a great source of knowledge and is a real asset to all of those in need.

Cheers!
MotoDan