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Maximum UART rate

For issues specific to the 8051 family, including compilers (e.g. Keil C51) and IDEs (e.g. uVision2).

Maximum UART rate

Postby sncuser » Thu Jul 31, 2014 6:06 pm

Is there a known maximum baud rate on the UARTs on the PPM(s) that use the C8051F120? I have to interface to a system that I thought was running at 38400, and just found out that it is actually running somewhere around 2Mbps! I'm going to try to get that rate lowered, but need to know how fast we can go.

Thanks.
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Re: Maximum UART rate

Postby Andrew » Fri Aug 01, 2014 8:52 am

Max baud rate is whatever the C8051 will support ... see C8051F120 datasheet.

The PPM B1 xtal is 24.5MHz, and the part can run at up to 100MHz. But so far, our customers have used it at 25MHz, i.e. at 1/4 of its full speed.

In terms of async serial comms, keep in mind that the CSK Bus is single-ended and not controlled impedance, so speeds of signals on the bus connector will be limited primarily by these factors. That said, running bidirectional async comms on the CSK at 1Mbps (e.g., on IO.4 & IO.5) with the C8051 has been done.
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Re: Maximum UART rate

Postby sncuser » Fri Aug 01, 2014 10:29 am

We are running it at 100 MHz, using the PLL. At least, I guess that qualifies as running it at 100 MHz.

So, asynch comms has never yet been done at 2 Mbps. Since initially posting this, I've found out that some other parts in the system are limited to 115K, so I believe we are going to have to reduce it, plus I would start to get a little worried about being able to service the comms at that rate along with everything else that the processor has to do. Although I guess if it has been done at 1 Mbps at a 25 MHz processor speed, then I guess we might be able to do 2 Mbps, simply based on processor speed.

I looked through the data sheet for the C8051F120 and didn't see anything mentioned about a speed limit on the UARTs, but I'll take another look.

Thanks!
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Re: Maximum UART rate

Postby Andrew » Fri Aug 01, 2014 1:53 pm

sncuser wrote:I looked through the data sheet for the C8051F120 and didn't see anything mentioned about a speed limit on the UARTs, but I'll take another look.
IIRC, you'll find in the Pumpkin EFFS-THIN docs, I ran SPI on PPM B1 at 12.25MHz, so max SPI clock is CLK/2. Dunno re the UART, but agreed, don't want incoming chars at > 1Mbps.
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