USART.Read 2-character limit?

General discussion relating to the library modules supplied with the compiler

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ggunderson
Posts: 3
Joined: Tue Dec 01, 2015 12:08 am

USART.Read 2-character limit?

Post by ggunderson » Tue May 17, 2016 9:53 pm

Hi, I have a question about the Swordfish USART read function when it comes to strings. It seems I can only read 2 ASCII characters from the USART.Read(myString) call. I'm using a PIC18F46K22 which has a 2-byte EUSART receive buffer. I've read most string receiving routines use a circular buffer to store incoming data, and I'd assumed the Swordfish USART library would implement a technique like that, allowing to receive strings of more than 2 bytes length. Unfortunately, the USART.Receive(myString) command becomes permanently blocking if I send a continuous stream of more than two bytes as a string.

But, the USART.Read() function works fine if I manually type into a terminal program (i.e., the character bytes are spaced out by keystroke interval). If I send those bytes continuously, it hangs.

This limitation doesn't depend on the baud rate. I can copy some code here, though I'm guessing there's something I might be missing or that someone else already knows the answer to receiving more than 2 characters as a string with this library module. It seems I should be able to beam a simple "Hello World" + terminator string as a single line transmission and have the USART routine get the whole string. So far all it's willing to receive is "H" + terminator or the terminator alone, if I send the item as a single transmission, though it works if I type each letter (and send them spaced out in time).

Any ideas?

Graham

ggunderson
Posts: 3
Joined: Tue Dec 01, 2015 12:08 am

Re: USART.Read 2-character limit?

Post by ggunderson » Tue May 17, 2016 10:19 pm

Look, I get to reply to my own post. The solution here is trivial but I'm sharing in case anyone finds this in the future. The PIC18F46K22 does have a 2 byte receive buffer. I got the string length limit issue by checking

If USART.DataAvailable = true Then ...

in the code, but then calling a few subroutines to prepare data for a response string. The time taken to do that was time enough for the receive buffer to overrun and halt receiving data. I solved the issue by calling

USART.Read(STR_RX)

directly after the If statement, to begin reading the buffer immediately.

Graham

Jerry Messina
Swordfish Developer
Posts: 1245
Joined: Fri Jan 30, 2009 6:27 pm
Location: US

Re: USART.Read 2-character limit?

Post by Jerry Messina » Wed May 18, 2016 12:49 am

For buffered serial input check out the isrrx interrupt module.

That adds an interrupt driven buffer to the uart input.

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