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boards:sbc:sbc_v2:start [2019/05/12 05:19]
b1ackmai1er [Features]
boards:sbc:sbc_v2:start [2019/07/13 06:23] (current)
b1ackmai1er [Board]
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 A description of the circuit operation can been seen here: [[http://​obsolescence.wixsite.com/​obsolescence/​the-n8vem-sbc|http://​obsolescence.wixsite.com/​obsolescence/​the-n8vem-sbc]] A description of the circuit operation can been seen here: [[http://​obsolescence.wixsite.com/​obsolescence/​the-n8vem-sbc|http://​obsolescence.wixsite.com/​obsolescence/​the-n8vem-sbc]]
 +
 +A video showing the construction of the SBC-V2-003 can be seen here: [[https://​www.youtube.com/​watch?​v=gXUt_EX-eo0|https://​www.youtube.com/​watch?​v=gXUt_EX-eo0]]
  
 The current version is SBC-V2-003A. A new version is under development and details can be found [[:​boards:​sbc:​sbc_v2:​sbc_v2-004|here]]. The current version is SBC-V2-003A. A new version is under development and details can be found [[:​boards:​sbc:​sbc_v2:​sbc_v2-004|here]].
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 Kicad: {{:​boards:​sbc:​sbc_v2:​ecb_sbc_v2-003a-20190306.zip|:​boards:​sbc:​sbc_v2:​ecb_sbc_v2-003a-20190306.zip}} Kicad: {{:​boards:​sbc:​sbc_v2:​ecb_sbc_v2-003a-20190306.zip|:​boards:​sbc:​sbc_v2:​ecb_sbc_v2-003a-20190306.zip}}
  
-{{https://​www.retrobrewcomputers.org/​lib/​plugins/​ckgedit/​fckeditor/​userfiles/​image/​boards/​sbc/​sbc_v2/​sbc-v2-003a.png?​direct&​800x474}}+[[https://​www.retrobrewcomputers.org/​lib/​exe/​fetch.php?​tok=0132fb&​media=https://​www.retrobrewcomputers.org/​lib/​plugins/​ckgedit/​fckeditor/​userfiles/​image/​boards/​sbc/​sbc_v2/​sbc-v2-003a.png|{{https://​www.retrobrewcomputers.org/​lib/​plugins/​ckgedit/​fckeditor/​userfiles/​image/​boards/​sbc/​sbc_v2/​sbc-v2-003a.png?​direct&​800x474}}]]
  
  
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 {{https://​www.retrobrewcomputers.org/​lib/​plugins/​ckgedit/​fckeditor/​userfiles/​image/​boards/​sbc/​sbc_v2/​sbc-v2-beep-mod.png?​nolink&​800 ​ }} {{https://​www.retrobrewcomputers.org/​lib/​plugins/​ckgedit/​fckeditor/​userfiles/​image/​boards/​sbc/​sbc_v2/​sbc-v2-beep-mod.png?​nolink&​800 ​ }}
 +
 +===== Speed switch modification:​ =====
 +
 +The following modification allows the cpu clock speed to be changed from full to half speed under program control. The purpose of this modification is to provide a level of compatibility with MSX applications while retaining the high speed capability. Specifically a 7.159Mhz crystal can be installed for normal operation including high density floppy disk support but be switched to 3.5795Mhz to drive the Sprite Colour Graphics board when required.
 +
 +This modification requires a 74LS74 to be modified and mounted on top of U4. Five leads are required to connect to other parts of the circuit. Suggested installation method:
 +
 +Prepare connection points for new clock source:
 +
 +  - Remove Z80 CPU U4 and fold out pin 6 and reinsert.
 +  - Remove 74LS244 U17 and fold out pin 6 and reinsert.
 +
 +Prepare a 74LS74 for mounting on 74LS10 U4
 +
 +  - Fold up pins 1-6 and 8-13 and snip off the ends leaving the stubs. Pin 7 and 14 remain for connecting to 74LS10 U4.
 +  - Solder a wire from pin 14 to pin 1 and then to pin 11 (brown wire).
 +  - Solder a wire from pin 4 to pin 9 (blue wire)
 +  - Solder wire from pin 13 to pin 5 (green wire)
 +  - Solder wire from pin 12 to pin 7 (yellow wire)
 +
 +Mounting 74LS74 and connecting flying leads on component side of board:
 +
 +  - Place the 74LS74 over 74LS10 U4 so only pin 7 and 14 are touching.
 +  - Solder pin 14's together and pin 7's together.
 +  - Solder a wire from pin 2 of the 74LS74 to pin 5 of 74LS174 U18 (purple wire)
 +  - Solder a wire from pin 11 of the 74LS74 to pin 12 of 74LS06 U26 (orange wire)
 +  - Solder a wire from pin 3 of 74LS74 to pin 13 74LS06 U26 (red wire).
 +  - Solder a wire from 7 of the 74LS74 to lifted pin 6 of 74LS244 U17 and then to lifted pin 6 of Z80 CPU U24 (yellow wire)
 +
 +Solder side connections:​
 +
 +  - Solder wire from pin 5 of CPU clock P4 to pin 13 74LS06 U26 (red wire).
 +
 +{{https://​www.retrobrewcomputers.org/​lib/​plugins/​ckgedit/​fckeditor/​userfiles/​image/​boards/​sbc/​sbc_v2/​sbc-v2-003-speedmod.png?​nolink&​800x424}}
 +
 +Software control
 +
 +Bit 4 of port 70h is used to select the clock speed. The normal condition is for Bit 4 to be set to 0 (low) and this will select the full clock speed of the installed crystal. Writing a 1 (high) to Bit 4 of port 70h will activate the 74LS74 clock divider. The speed switch is currently not supported in ROMWBW. Use of the RTC or speaker will reset Bit 4 to 0 i.e. full speed operation. Further, precalculated software delays or delays based on the initial speed detected at boot time do not take into account the possibility of the speed changing.
 +
 +Demonstration code:
 +
 +<​code>​
 +1 OUT 112,0
 +2 PRINT "​NORMAL SPEED"
 +3 GOSUB 10
 +4 OUT 112,8
 +5 PRINT "SLOW SPEED"
 +6 GOSUB 10
 +7 END
 +10 FOR X=1 TO 64
 +20 PRINT "​.";​
 +30 FOR Y=1 TO 256
 +40 NEXT Y
 +50 NEXT X
 +60 PRINT
 +70 RETURN
 +</​code>​
 +
 +{{https://​www.retrobrewcomputers.org/​lib/​exe/​fetch.php?​media=boards:​sbc:​sbc_v2:​sbc-v2-003-speedmoddemo.gif?​linkonly|Demonstration video}}
  
  
boards/sbc/sbc_v2/start.1557652743.txt.gz · Last modified: 2019/05/12 05:19 by b1ackmai1er
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