On 9/5/2018 3:25 AM, smf wrote: > On 05/09/2018 08:54, smf wrote: >> >> "On-chip baud rate generator: 15 programmable baud rates from a >> standard 1.8432 Mhz external crystal (50 to 19,200 baud)." >> >> "External 16x clock input for non-standard baud rates (up to 125Kbaud)" >> > I forgot to mention, later in the datasheet it's clear that a crystal > and clock are different things. They are indeed. A crystal is a simple piezoelectric device, which requires a circuit to cause the harmonic oscillation. An oscillator is a crystal and the required electronics to perform that activity. > > > The block diagram shows that the /16 is always active, the baud rate > generator takes XTAL1 & XTAL2 and bits 0-4 of the control register. I > don't know what happens electrically when you switch the internal baud > rate generator off, which the datasheet indicates the XTAL2 must float > and it's not floating because it's hooked up to a crystal. You can't really turn it "off" Think about the 16 bps rates as simply a lookup to some divisors: 0000 = 1 0001 = 1843200/(50*16) = 2304 0010 = 1843200/(75*16) = 1536 0011 = 1843200/(109.92*16) = 1048 etc. > > It's also not clear how bidirectional the RxC pin is either, when > using the on baud generator the block diagram just shows that it gets > hooked up internally to the RxC pin. So if you had something supplying > an RxC input as well then it's not clear that the external clock would > be ignored. The direction of the RxC pin is set via bit 4 of the control register. if 0, RxC is an input, and is used to drive the receiver logic. If 1, it is an output and will represent the output of the divisor above (16* the receiver bps rate) The idea was to allow split bps rates (600bps send, 300bps receive). No one actually used it, but it is supported. Jim > > > -- Jim Brain brain@jbrain.com www.jbrain.comReceived on 2018-09-05 17:03:24
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