635 serial comms

[cjs]

i've got conflicting information on the serial protocol for the 635 lcd module. i'm interfacing over ttl serial to an atmega128, do i program the crc error checking protocol into the firmware or is that an internal function of the microprocessor on the lcd, in which case the avr firmware checks for the status of the serial packet sent from the lcd unit??

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[CF Tech]

The CFA-635 communicates using packets.

For it to be a packet, it has to have the correct CRC.

So you will have to be able to generate the CRC in the AVR to get the CFA-635 to respond.

 

[cjs]

so in that case, the avr sends the data packet w/ the crc result, if the packet recieved by the lcd has the wrong crc result meaning the data is corrupted, there will be no response?? or does it reply back w/ a "00", "01" "10", or "11" as per the manual spec's on handshaking??

which poses another question, what is the most common way to structure firmware for communicating to this lcd, usually i use interrupts when the rx or tx buffers have data but due to the handshakes as well as the 250ms watchdog timer for hardware errors it seems like it may be better to moniter the handshakes cyclically rather than in the interrupt itself....i'm just wondering what other people have had success with.

 

[CF Tech]

There cannot be a "packet recieved by the lcd has the wrong crc" if the CRC is not correct, it is not a packet. By definition, the packet must have the right CRC.

If you do send a sequence of bytes that would otherwise be a packet except for the bytes in the CRC location not being the CRC, the module will discard the bytes in exactly the same manner as random data.

In short, the module will ignore a packet with a bad CRC. (Dang, now I said it!)

There should be no "handshaking". You can send your packet to the module at full speed. You need to be prepared to receive packets from the module at full speed.

 

[CF Tech]

Please take a look at "CheckForPacket()" and "SendPacket()" in the 635_WinTest code:

http://www.crystalfontz.com/software/635WinTestindex.html

 

[cjs]

found it....for future referance the "packet_debug.c" file gives a better overview of how to structure the host serial transfer code, thank you for the assistance