Programming Baud Rate


I am professor of Computer Integrated Electronics / Mechatronics at Sierra College in Rocklin California and I use the PICAXE in my embedded systems class. The topic for this week's class was asynchronous serial communication.

I simply wanted to tie the topic of baud rate to the programming process for a PICAXE and the reason why the resistors are required in the download circuit. I suppose I could have just connected up one of our digital storage oscilloscopes and captured the signal.

That said, I appreciate the need to keep some of the information proprietary.


New Member
The resistors are needed to convert RS232 levels to logic levels. They are a 'poor mans' max232 (and a lot cheaper). The series resistor limits the current. +12V in becomes +5V inside the chip due to current clamp diodes. -12V in becomes 0V due also to current clamp diodes. The 10k to ground keeps the pin grounded when the programming cable is disconnected.

Incidentally, the 22k/10k network also works as an input to other chips, eg 74HC series work just fine with this as well.


Thank you for the reply.

I know about the resistors, that is part of my class lecture.

My question was regarding the serial communication baud rate.


Senior Member
My suspicion - but I highly recommend verifying that with a scope or logic analyzer - is that at least for the older parts like 08M, 18X and so on the download data rate is the same as the sertxd/serrxd data rate for that particular device. E.g. 4800 baud for an 08M running at 4 MHz.


Technical Support
Staff member
No secret, anyone with a scope or logic analyser could work it out!

This is relatively slow by modern standards but was a good compromise between speed, processing time between bytes (to actually save the downloaded data), and what the early original PICAXE (4MHz) devices could keep up with reliably.

After each byte is downloaded it is echoed back to the computer so that the computer can verify each byte was reliably received.
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