Instant Interrupts - Revisited

[Demon]

Darrel,

Any obvious reason why this program takes over 10 seconds to start up on a PIC 18F4550?

If I comment out the Interrupt settings at the top (includes and asm) and the ReceiveUSART handler further below, it takes less than 2 seconds.

Robert

[Darrel Taylor]

Not sure about this, but gotta start somewhere.

The RX_INT is being ENABLEd before the USART registers are set-up. Or, more accurately, since HSERIN/OUT are used in the program, PBP initializes the USART first, then RX_INT is enabled, then the USART registers are changed.

It may be causing a false interrupt that sends it to the ReceiveUSART: handler, which is expecting to see 2 bytes being received, that were never sent. So it sits there.

Try using this...

DEFINE HSER_CLROERR 1  ' Hser clear overflow automatically 
DEFINE HSER_RCSTA 90h  ' Hser receive status init 
DEFINE HSER_TXSTA 24h  ' Hser transmit status init 
DEFINE HSER_SPBRG 0    ' Hser spbrg init 

While PIR1.5 = 1     ; clear the buffer
    HSERIN [varByte]
Wend

@ INT_ENABLE  RX_INT     ; enable RX interrupts

In place of..

@ INT_ENABLE  RX_INT     ; enable RX interrupts


DEFINE  HSER_SPBRG  0               ' 1250 KBAUD @ 20 MHz (CPU)

        BAUDCON.3 = 0               ' BRG16 Select 8 bit baud rate generator

        TXSTA.5 = 1                 ' TXEN  Enable USART transmitter
        TXSTA.4 = 0                 ' SYNC  Asynchronous communication
        TXSTA.2 = 1                 ' BRGH  High speed baud rate

        RCSTA.7 = 1                 ' SPEN  Enable USART receiver
        RCSTA.4 = 1                 ' CREN  Enable continuous receive

[Demon]

Nope, wasn't it. But as I was putzing around trying other stuff I noticed a few attempts were within 2 seconds, but for no apparent reason. For some reason it appeared to be an unstable setup and I remembered that unset pins can do this. And that's when I remembered this from page 235 of the 18F4550:

-----------------------------------------------------------------------
The pins of the Enhanced USART are multiplexed with
PORTC. In order to configure RC6/TX/CK and
RC7/RX/DT/SDO as a USART:
• bit SPEN (RCSTA<7>) must be set (= 1)
• bit TRISC<7> must be set (= 1)
• bit TRISC<6> must be cleared (= 0) for
Asynchronous and Synchronous Master modes
or set (= 1) for Synchronous Slave mode
-----------------------------------------------------------------------

I had ignored this paragraph 'cause I had never seen a need for these resistors, I thought it was only for the Enhanced USART. So I added the 4K7 resistors on TX (pulldown) and RX (pullup) and everything works fine, even with my old code.

Thanks D!

Robert

[Demon]

Well, I misunderstood what I was supposed to do but when I removed the resistors and put in the TRIS statements it took 9 seconds again. So I set the port pins to 0, same thing, I set them to 1, same thing.

It works properly with just the 4K7 resistors.

EDIT: I tried pulling both pins down and that works as well.

Robert