On 09/06/2018 08:20 PM, Mia Magnusson wrote: > Den Thu, 6 Sep 2018 19:56:26 +0200 skrev Gerrit Heitsch > <gerrit@laosinh.s.bawue.de>: >> On 09/06/2018 07:40 PM, Nils Eilers wrote: >>> On Wed, 5 Sep 2018 16:39:44 +0200 >>> Gerrit Heitsch <gerrit@laosinh.s.bawue.de> wrote: >>> >>>> The 6551 doesn't need capacitors or resistors. All you need is to >>>> connect the crystal to the two XTAL pins and you're done. >>> >>> The W65C51N datasheet shows a schematic at page 19 with a 30 pF >>> capacitor from XTALI to GND and a 1 M-Ohm-resistor from XTALI to >>> XTALO. >> >> Well, that's the 65C51... I was talking about the 6551 and its >> datasheet doesn't mention anything of the sort. >> >> And if you look at the schematics of the Plus/4, you'll see that >> there is only the crystal. > > Not deriving the baud rate clock from the system clock must have added > to the cost. Especially since back then, they used a rather large crystal: https://www.thefuturewas8bit.com/media/sitebuildercontent/sitebuilderpictures/P4before.jpg (See on the right side) > Of course you'd need a divisor that isn't as nice as 16 to get correct > baud rates with a clock that can also be used to generate a NTSC or PAL > color signal. If the 6551 addition had been in the plans when TED and > CPU were designed for the Plus/4 series, they might have had that > buildt in. Maybe.. but that would need something like a PLL, neither PAL nor NTSC clock will divide easily into 1.8432 MHz. > Btw a "6552" which can take color clock might had been something they > could sell anyways. They had something like this in the PC... I have seen ISA cards that didn't have a crystal and one of the UARTs was a special one (Winbond 86C451, you can find the datasheet online). It used the 14.318 MHz from the mainboard to generate the baud clock for itself and a possible second UART on the same card. GerritReceived on 2018-09-06 21:00:21
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