Date: Fri, 25 Aug 1995 00:11:20 +0200 (MET DST) From: J Wunsch <j@uriah.heep.sax.de> To: freebsd-hackers@freebsd.org (FreeBSD hackers) Subject: Re: ISDN Anyone? Message-ID: <199508242211.AAA17399@uriah.heep.sax.de> In-Reply-To: <199508242026.NAA08474@gndrsh.aac.dev.com> from "Rodney W. Grimes" at Aug 24, 95 01:26:37 pm
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As Rodney W. Grimes wrote: > > I am not so worried about the National parts, it is the other manufactures, > as you point out later, that have different upper clock rates. I seem > to recall the TMS version topping at 5MHz, but maybe confusing that with > the Zylog SIO chip. Incidentally, i've got a pile of ancient photocopied Z80 docs on my desk: the SIO could handle up to 400 ns TxC/RxC cycles, or 2.5 MHz. The usual limitation to async rates of only 9600 Baud resulted out of 2.5 MHz pre-divided by 16 in the CTC "baud rate generator", and by using the 1:16 pre-divider in the SIO to avoid synchronization problems. Anyway, it seems impressive from today's point of view that you could go much beyond that... (and even got SDLC/HDLC). Even using the 1:16 divider at maximal clock rate would still yield 150 kbps. (And the Z80 SIO came with a 3-byte FIFO.) :-) 16C451 -- is this the PLCC-packaged chip (as opposed to the usual DIL-packaged 16[45]50's)? > I can tell you why they upped fmax to 24Mhz, that is the same clock needed > by most floppy controller chips, so now you can run both chips off the > same crystal on a multipurpose card. This is quite common in the Winbond > and SMC chips, there is an additional divider inside the chip that cuts > this down for the uarts, but uses it raw for the floppy controller. Look > on any winbond based atio card and you will see 1 24Mhz crystal. > > The 451/551 integrate some stuff, but I can't recall what ``stuff''. Is it possible to acces and re-program the pre-divider on this chip? -- cheers, J"org joerg_wunsch@uriah.heep.sax.de -- http://www.sax.de/~joerg/ Never trust an operating system you don't have sources for. ;-)
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