Packet over serial: Header and values collisions

ELCouz · Joined: 18 Jul 2007 Posts: 427 Location: Montreal,Quebec

I need to send very simple packets from PC to PIC very fast! (100 times per sec.)

Interfacing PC to PIC is not the problem.

I will have some conflicts with the packet start header and the packets values.

The 16 values are 12-bit.

So I thought of doing:

newguy · Joined: 24 Jun 2004 Posts: 1908

Choose unique START and STOP ASCII characters. The only legal place they can occur is at the start and stop, respectively, of a packet.

Legal packet:

[START] [n bytes of data] [STOP]

If, in the middle of your packet you need to transmit a START or STOP (as part of your data stream), insert an escape sequence instead. Take the standard ASCII escape - 0x1B.

0x1B 0x1B = one byte that is 0x1B
0x1B 0x1C = one byte that is START
0x1B 0x1D = one byte that is STOP

You can make your scheme more robust by also transmitting the number of bytes in the stream and/or a checksum.

ELCouz · Joined: 18 Jul 2007 Posts: 427 Location: Montreal,Quebec

newguy · Joined: 24 Jun 2004 Posts: 1908

Doesn't have to be ASCII. Just set aside two unique characters (start/stop) and a further "escape" character that allows you to insert the start/stop within your datastream.

gpsmikey · Joined: 16 Nov 2010 Posts: 588 Location: Kirkland, WA

Be very careful transferring raw bytes over the serial line - especially if one side is the PC. You often don't have access to the low level drivers for the PC and can get unpleasant surprises when the data happens to look like special characters to the PC (CR, LF and others come to mind, having been bitten by that in the past where raw 0x0D got an automatic LF (0x0A) appended by the driver (this was some time ago). I would test a full range of raw data through the PC just to make sure it is not going to "help" you.

mikey
_________________
mikey
-- you can't have too many gadgets or too much disk space !
old engineering saying: 1+1 = 3 for sufficiently large values of 1 or small values of 3

ELCouz · Joined: 18 Jul 2007 Posts: 427 Location: Montreal,Quebec

Ttelmah · Joined: 11 Mar 2010 Posts: 19536

Are you 'locked' to 8bit serial?.

If not, consider using 9bits. Use bit 9 as the 'marker'. So start header with bit 9 set, then data packet with bit 9 off.

This is quite common on some RS485 protocols, where the 'target address' is sent with bit 9 on.

ELCouz · Joined: 18 Jul 2007 Posts: 427 Location: Montreal,Quebec

gpsmikey · Joined: 16 Nov 2010 Posts: 588 Location: Kirkland, WA

I was looking into using the 9th bit to indicate it was an address quite a while ago when I found one of the new (at the time) 8751 family (FX?) supported the 9th bit as an address flag according to the data sheet. No other info so I called the Intel FAE - his response was "it does?? - I'll have to check it out and get back to you on that". Still waiting for that call back :-) I have seen some uarts that do handle the 9 bits, but I would definitely check out the hardware I was going to use before I assumed that it did support 9 bits.

mikey
_________________
mikey
-- you can't have too many gadgets or too much disk space !
old engineering saying: 1+1 = 3 for sufficiently large values of 1 or small values of 3

temtronic · Posted: Thu Jan 22, 2015 1:31 pm

Motorola has(had) an addressable UART 'MC14469' used the 9th bit. Might be useful info in the app note. been 15 years since I used them though.

Jay

ELCouz · Joined: 18 Jul 2007 Posts: 427 Location: Montreal,Quebec

ELCouz · Joined: 18 Jul 2007 Posts: 427 Location: Montreal,Quebec

Ttelmah · Joined: 11 Mar 2010 Posts: 19536

The PIC UART can handle 9 bit data. You normally write this with the parity bit (since except for some of the DsPIC's, the PIC does not generate parity in hardware), but the CCS functions offer the ability to send the 9th bit if you use a 16bit value to the routines.

Even the earliest PC can also generate 9bit data, but it takes a bit of coding. You 'cheat', and use a look up table to code for every byte value, whether even, or odd parity would generate a 0 or a 1 in the 9th bit, and then toggle the UART setup for each byte.

Many modern PC's allow 9bit to be directly sent. It's only older ones with a plain UART that can't. Later boards use an emulation of the 16C950, that has a micro-controller mode giving 9bit support.

temtronic · Posted: Thu Jan 22, 2015 3:48 pm

re: ...
The MC14469 ... well ... is old! ...

gee , thanks, makes me feel like a dinosaur....

I do have equipment running at 24 baud( yes, 24 bits per second) for the past 20 years on direct wire over 15 miles( yes, miles) of copper.

Don't know if the PC can handle 9 bits though, might need to make a 'bridge' unit. Who knows what's inside a PC these dayze !

Jay

asmallri · Joined: 12 Aug 2004 Posts: 1635 Location: Perth, Australia