the diy-sbc

[John S Gwynne]

--------

   In message <m0r0wCf-000uIZC at twisto.eng.hou.compaq.com> , you write:
 
| 1)  what is the circuitry connected to IC9 pin1?  Debounce for the switch?

The initial reset must be asserted for 100ms. It works but it can
be replaced by a "reset generator" with fewer external parts. See below...

| 2)  what are your pal equations?

appended to the end of this e-mail.

| Anyway, it seems complete, what are your plans for enhancements?  I mean on 
| this board, not plans for expansion boards...

1) ic10 and most of the discrete parts attached to it will be replaced with a
MAXIM "watch-dog/battery switch over/reset generator". I hope to order this
next week...

2) I need a few wait states for the external bus that will be on the
expansion board. Right now this should just be an equation change to ic11, but
I may add one connection between E1 and ic11.

3) In hind sight, I think ic13 and ic15 (the battery backed-up ram) should
be put on the expansion board and 2 128kx8 SRAM chips put on this board. 
Originally, I was going to put 0.5 or 1 Meg of DRAM (SIMMs) on the expansion
board, but this seems like too much over kill now. I intend to put 2 128kx8
SRAM chips on my expansion board.

4) A connection will be made between the reset generator and IC19 to ensure
that the battery backed-up RAM can not be selected during the first few
clock cycles of a power-on reset. 

5) I made the following pin swaps to minimize wire path lengths.
	IC7:
	(pin as shown on the schematics),(was changed to new pin number)
	2,9 3,8 4,7 5,6 6,5 7,4 8,3 9,2 18,11 17,12 16,13
	15,14 14,15 13,16 12,17 11,18

	IC2:
	11,17 13,15 15,13 17,11 9,3 7,5 5,7 3,9
	
	IC3:
	11,17 13,15 15,13 17,11 9,3 7,5 5,7 3,9
	
6) The interrupt priority encoder and heart-beat 100mS interrupt should be on
the cpu board to make it more stand-alone. But, (1) there are a limited
number of pins free on E1 for other external interrupts and (2) not a whole
lot of space left. Expect pins 23,24, and 25 of ic1 and pin 30 of ic17 to be
routed to E1 with maybe pull-up resistor for stand-alone mode.

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
GAL equations follow (all GAL's are 15nS)
================ cut here =================
ic9 file

chip ic9 gal16v8a

;1      2       3       4       5       6       7       8       9       10
R1	nc	nc	nc	R	nc	nc	nc	nc	GND
nc	nc	nc	RESET1	W	nc	!RESET	!HALT	nc	VCC

equations

RESET = !R1
HALT = !R1
RESET.OE = !R1
HALT.OE = !R1
RESET1 = RESET.PIN
W = !R
================ cut here =================
ic11 file

chip ic11 gal16v8a

;1      2       3       4       5       6       7       8       9       10
CLK24	!AS	A23	A22	A21	NC	NC	NC	NC	GND
NC	NC	NC	NC	NC	CLK6	!DTACK	!VPA	CLK12	VCC

equations

CLK12 := !CLK12
CLK6 := !CLK6 & CLK12 | CLK6 & !CLK12
DTACK = AS & !A23 | AS & !A22 | AS & !A21
VPA = AS & A23 & A22 & A21
================ cut here =================
ic19 file

chip ic19 gal16v8a

;1      2       3       4       5       6       7       8       9       10
NC	!UDS	!LDS	!AS	A23	A22	A21	R	NC	 GND
NC	NC	NC	!U_RD	!U_WR	!U_CS	!HBRAM	!LBRAM	!EPRM	VCC

equations

EPRM = AS  &  R & !A23 & !A22 & !A21
LBRAM = LDS &      !A23 & !A22 &  A21
HBRAM = UDS &      !A23 & !A22 &  A21
U_CS  =            !A23 &  A22 & !A21
U_WR  = LDS & !R & !A23 &  A22 & !A21
U_RD  = LDS &  R & !A23 &  A22 & !A21
================ cut here =================




                                       John S Gwynne
                                          Gwynne.1 at osu.edu
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