Baud rate calculation

[Demon]

Hi,

I have a 16F876A transmitting via HSEROUT to a 18F4550 with a 20MHz crystal and I'd like to use the maximum speed possible.
http://ww1.microchip.com/downloads/e...Doc/39582b.pdf

I select High Speed (BRGH=1) on table 10.1 on p.115, so I only have the option of SYNC=0 (asynchronous). Asynchronous table 10-3 on p.116 gives a KBAUD rate of 312.5 for a 20 MHz speed and a SPBRG value 0.

I have absolutely no idea how they got from this formula FOSC/(16 (X + 1)) using the settings I chose. Anyways, how does this relate with the HSER_BAUD command? KBAUD means kilobaud in my mind, that would mean 312.5 x 1000, that value seems waaaaaay too high for me. I wasn't expecting anything higher than 115200 or so based on experience with modems.

I checked the datasheet for the 18F4550 (table 20-3 on p.245) and that's exactly the top end:
http://ww1.microchip.com/downloads/e...Doc/39632c.pdf

So now I'm confused on what setting to take to get the 2 chips working reliably. I'd really like to understand this 'cause I might have to change chip later.

Robert

[mister_e]

Look at that...
http://www.picbasic.co.uk/forum/atta...6&d=1118138371

Yeah the same placed on another website but for PBP... from the same guy

[Darrel Taylor]

I was just going to mention your calculator Steve. Beat me too it.

Robert,

I think you're looking at the wrong table. 10-3 is for BRGH=0. For BRGH=1, see table 10-4.

@ 20mhz, with BRGH=1, and SPBRG= 0, the baud rate is 1250K, that's 1.25 million bits per second. And the formula works with that one.

[Demon]

Crap, and I had checked my work too. Getting old...

Steve's utility gave this (real cool, can check calculations instantly, saves me from typing that into an Excel sheet):
DEFINE HSER_RCSTA 90h
DEFINE HSER_TXSTA 24h
DEFINE HSER_SPBRG 10 ' 115200 Bauds

I'm not a fan of all-encompassing hex values like that, I like to know what is happening. Is there a significant difference in performance if I set the bits manually like this?
DEFINE HSER_SPBRG 10 ' 115200 Bauds
RCSTA.7 = 1 ' SPEN Serial port enable
RCSTA.4 = 1 ' CREN Continuous receive enable
TXSTA.5 = 1 ' TXEN Transmit enable
TXSTA.4 = 0 ' SYNC Asynchronous communication
TXSTA.2 = 1 ' BRGH High speed baud rate

I like comments, it helps make up for a miserable memory. I can toggle stuff ON and OFF easily that way. I usually ignore default values, but I make exceptions for important and relevant settings like SYNC in the example above.

Robert

[Demon]

Steve, any chance of letting the baud rate utility accept 1250000? That is a valid speed according to the datasheets, but the utility does not accept it.

Robert

[Demon]

These values work in the utility:
FOSC=20
SPBRG=0
20/16(0+1) = 1.250 = 1250 KBAUD


These don't:
FOSC=48
SPBRG=0
48/16(0+1) = 3.000 = 3000 KBAUD

Don't you hate 18F MCUs?

Robert