Help: How to receive serial data without losses

[FoxtrotMike]

Hi:

I'm working on an application that requires to receive serial data, process it (som XOR operations), and then send it back. Also, i need to send a synchrony sequence in between, but without losing any bytes. This is my attempt at coding:

[PCM programmer]

Basic suggestion -- use a real receive buffer.
See the CCS example file, Ex_sisr.c. Here's the file location:

[FoxtrotMike]

Thanks... I'll look it up

[SET]

In general what your doing looks ok. But what happens if your the number of incoming bytes is not divisible by 4?. Then your 4 byte buffer will never be filled and you wont get those ones back - is this what you saw in the sim?

[Bill Boucher]

I was reading somewhere that the compiler will automatically disable interrupts while doing certain functions that are timing sensitive such as putc(). That said, when the program tries to transmit 4 bytes in a row using sequential putc() lines, could it be that interrupts are off throughout them and that the incoming data interrupt is not being immediately serviced and so the 2-byte receive buffer overflows?

I didn't look at the 18F252 datasheet to check the uart pin numbers but I assume you're using the hardware Tx & Rx pins. The Tx buffer is also 2 bytes deep so when the four putc() lines run, the first 2 execute very fast because all they do is load the bytes into the hardware buffer, but the next 2 putc() lines each have to wait until the buffer is ready to accept more data, that is until the first 2 bytes are done transmitting. That causes a couple ms delay. Could a byte or two being received during this time be dropped? I wonder.

If you create a r_buffer[] array var, and a t_buffer[] array var, then set up the int_rda and int_tbe interrupts with code that simply moves hardware-received data into r_buffer[] and data from the t_buffer[] to the hardware-transmit register (using putc) just one byte at a time when the interrupts occur, then you won't lose any data. Pointers are used to tell the ISR's where to read/write data from/to.

Example:

[temopic]

Can I use similar buffers for SPI interrupts?

Ex_sisr.c

[Douglas Kennedy]

Circular buffers are useful for any I/O both with and without flow control.
Flow control is always the complete answer.
The isr driven circular buffer is extremely useful and often the only way to get RS232 to work reliably. However the circular buffer is still no more than a shock absorber in that is smooths out bursts of Rs232 data both in and out(if you use it for output as well) and allows more flexiblity in that the PIC code doesn't need to be conformed to such a tight loop around kbhit(). Without flow control the PIC design must be able to do its work and not mess up the average ( no overflows of buffers on either input or output). Consider an example where for every character received two are sent out. If there is no flow control and the input is a continuous unbroken stream of characters then an output buffer overflow is guaranteed ( assuming input baud rate is equal to the output baud rate).

[temopic]

Thank you Douglas
Could some body help make circular buffers for SPI in and out?

My code seems to get stucked after a SPI interrupt not allowing int_rda arrive

[Douglas Kennedy]

The examples above in this code show a circular buffer. I wouldn't use modulo % division. A simple test of the counter to see if it has reached the end of the buffer and a reset to zero if true is best practice. CCS has examples to look at that plus with the code above you should enable you to make progress.

[temopic]

thanks again douglas