Are you b building or thinking of building an Acorn 6502 BBC B/Master
Posted: 20 Dec 2018 13:55
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Ideas and Musings about Design & Building a homebrew 6502 Acorn BBC Computer
These are just ideas and thoughts and should perhaps be taken with a little pinch of salt. But they are a starting point for anyone possibly thinking of such an endeavour
Design ideas:
CPU:
As in the original a 6502 , but a newer CMOS 65C02 such as the R65C02Pxx,W65C02Pxx or even a W65C816 running at up-to several times the original 2MHz speed.
Speed. : Clock Speed could be switched/varied from 2MHz to as high as 16MHz ??
Memory :
Fast SRAM and EPROM/EEPROMs would be used to allow for much faster clock speeds. SRAMs such as UM61256/UM61512s 15ns type and 27C256-45 45ns are examples. Giving 128K SRAM or more.
Video :
As in the original machine a Motorola 6845 CRTC would be used (HD45605,R6545 etc) in conjunction with Dual Port SRAM such as IDT7007 (32k x 8 bit). This would allow the CPU and Video circuit to work independently. Allowing much higher CPU clock speeds without causing video disruption.
Sound:
As in the original a SN76489 would be implemented but also a YM2149 , SAA1099 or even speech synthesiser giving multiple sound generators and numerous sound channels, with stereo output. As these chips are cheap this could be done relatively inexpensively.
I/O
As in the original , namely 6522s,68B21,68B50,68B54setc to give compatibility with the original BBC B / Master . Floppy disc controllers could also be kept . This would probably means most I/O would be done with a relatively low clock speed of 2Mhz or possibly 4MHz ? Glue logic such as CPLDs (EPM7128,EPM7032s) or GALS could be used for the function of I/O chip selection, these would obviously need programming.
Teletext :
Again as in the original .
Glue logic :
Glue logic would be needed to interconnect all the various ICs, components this was mainly done by old TTL ICs and some dedicated ICs in the original. These could be replaced with more modern (programmed) GALs & CPLD (Complex Programmable Logic) in order to be more efficient . This would do such functions as Memory Mapping / Selection, I/O selection and various other low level functions. It could also be used to create the ‘Tube’ 1MHz (or More) to allow a second CPU to be connected.
Keyboard/ Mouse
This could be kept as original or a PS/2 or even USB keyboard connected with the aid of a Modern MCU ? I’ve seen several PS/2 circuit using MCUs so that should be relatively easy to implement. A USB keyboard would be more complex, but could be done.
Hard Drive / CF Card (Storage)
A hard-drive interface could be added using a PATA type . I’ve seen this done with Intel 82C55s and I’m sure there’s plenty of schematics for a CF Card interface on the web.
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Ideas and Musings about Design & Building a homebrew 6502 Acorn BBC Computer
These are just ideas and thoughts and should perhaps be taken with a little pinch of salt. But they are a starting point for anyone possibly thinking of such an endeavour
Design ideas:
CPU:
As in the original a 6502 , but a newer CMOS 65C02 such as the R65C02Pxx,W65C02Pxx or even a W65C816 running at up-to several times the original 2MHz speed.
Speed. : Clock Speed could be switched/varied from 2MHz to as high as 16MHz ??
Memory :
Fast SRAM and EPROM/EEPROMs would be used to allow for much faster clock speeds. SRAMs such as UM61256/UM61512s 15ns type and 27C256-45 45ns are examples. Giving 128K SRAM or more.
Video :
As in the original machine a Motorola 6845 CRTC would be used (HD45605,R6545 etc) in conjunction with Dual Port SRAM such as IDT7007 (32k x 8 bit). This would allow the CPU and Video circuit to work independently. Allowing much higher CPU clock speeds without causing video disruption.
Sound:
As in the original a SN76489 would be implemented but also a YM2149 , SAA1099 or even speech synthesiser giving multiple sound generators and numerous sound channels, with stereo output. As these chips are cheap this could be done relatively inexpensively.
I/O
As in the original , namely 6522s,68B21,68B50,68B54setc to give compatibility with the original BBC B / Master . Floppy disc controllers could also be kept . This would probably means most I/O would be done with a relatively low clock speed of 2Mhz or possibly 4MHz ? Glue logic such as CPLDs (EPM7128,EPM7032s) or GALS could be used for the function of I/O chip selection, these would obviously need programming.
Teletext :
Again as in the original .
Glue logic :
Glue logic would be needed to interconnect all the various ICs, components this was mainly done by old TTL ICs and some dedicated ICs in the original. These could be replaced with more modern (programmed) GALs & CPLD (Complex Programmable Logic) in order to be more efficient . This would do such functions as Memory Mapping / Selection, I/O selection and various other low level functions. It could also be used to create the ‘Tube’ 1MHz (or More) to allow a second CPU to be connected.
Keyboard/ Mouse
This could be kept as original or a PS/2 or even USB keyboard connected with the aid of a Modern MCU ? I’ve seen several PS/2 circuit using MCUs so that should be relatively easy to implement. A USB keyboard would be more complex, but could be done.
Hard Drive / CF Card (Storage)
A hard-drive interface could be added using a PATA type . I’ve seen this done with Intel 82C55s and I’m sure there’s plenty of schematics for a CF Card interface on the web.
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