USART at very hi speed 18F6520

[TopFuel]

I have a RF transceiver demo board that uses a Pic 18F6520 as the micro,

The RF chip has a baud rate of 1.536Mbps. The demo board uses a 12.288Mhz crystal and the PLL clock option. This makes the clock freq 12.288Mhz instead of it being divided by 4 like normal. This allows the Uart to be run at 1.536Mhz. Only problem is that the effective clock rate is 49.152Mhz which is over the max of 40Mhz, so they lowered the supply v to 4.5V. It seems to works so I cannot argue that. Not a good idea though.

I am trying to develop a simple Wireless presence detector. The transmitter will send out a repeating data pattern, if the receiver see the pattern, it sets an output. So far I have the TX part working using the hardware uart Confirmed with the scope. I have written the RX code using the Uart interrupt. I ran the code with ICD2 debugger, I can see that I am receiveing some data but after a few bytes the interrupt routine stops being called and the OERR bit is set. The data that I am sending is 0x55 5 times then some data patterns for a total of 29 bytes. The Irq code sucessfully captures the 5 0x55's and 1 byte of the data then stops.

I changed the baud rate of both TX and RX to 576000 and the code worked.

I did some math and as I figure, since the instruc clock freq is 12.288Mhz that means the instruction time is about 81nsec. If the Uart is setup at 1.536Mbps then the RDA interrupt should occur every 651 * 8 nsec or 5.208usec. In that time I should be able to execute 72 instructions. My whole ISR is only 55 instructions, however the longest path in the ISR is probably only 30 instructions. I have no other IRQ's enabled. I cannot figure out why I get OERRs.

Any help or troubleshooting techniques wouild be greatly appreciated.

[PCM programmer]

[TopFuel]

Looks like the compiler adds 22 more instructions, That could do it,

How can I make it faster?



0008: MOVWF 05
000A: MOVFF FD8,06
000E: MOVFF FE0,07
0012: MOVLB 0
0014: MOVFF FE9,0D
0018: MOVFF FEA,08
001C: MOVFF FE1,09
0020: MOVFF FE2,0A
0024: MOVFF FD9,0B
0028: MOVFF FDA,0C
002C: MOVFF FF3,14
0030: MOVFF FF4,15
0034: MOVFF FFA,16
0038: MOVFF 00,0F
003C: MOVFF 01,10
0040: MOVFF 02,11
0044: MOVFF 03,12
0048: MOVFF 04,13
004C: BTFSS FA3.5
004E: GOTO 0058
0052: BTFSC FA4.5
0054: GOTO 009E

[PCM programmer]

I'm not a specialist with high speed interrupts on the 18F PICs. Either
Ttelmah or ckielstra, or maybe some others could explain it to you.

[treitmey]

I'm just taking a swag here. but if the OERR is showing up, your not emptying the USART fast enough, right. So if the baud is fixed, reduce the byte count and have an inter-character timing.

USART is still running fast.
Only send 2 bytes,...that will fit in the USART, and not set OERR
Then wait some x amount of time before sending another 2 bytes.

During time x, (on the pic) you will be retrieving the data from the USART.



also, we had a chat about running past 40Mhz, The consensus seemed to be that if this is a production board, don't do it. The chip production may change slightly and you then may not find parts to build the board at desired speed.

[asmallri]

Here is how I would handle this:

Write the ISR in assembler. Use global variables in the handler located in the access bank and place your temporary variables in the access bank. Place the code directly at the HP interrupt vector address. You will be responsible for saving and restoring any registers your assembler code uses. You may need a large ring buffer depending on how long it takes your mainline to drain the ring buffer. If this is the case you will need to use an FSR pointing to RAM outside the access bank forcing you to save and restore the FSR.

Tell the compiler you do are not using HP interrupts.

In the mainline manually setup to the PIC for supporting the HP interrupt.
_________________
Regards, Andrew

http://www.brushelectronics.com/software
Home of Ethernet, SD card and Encrypted Serial Bootloaders for PICs!!

[ckielstra]