BB68008, Barebones 68008 SBC

Introduction

BB68008 is similar to BB80 and BB65 consisting of MC68008 CPU, 128K RAM, and 22V10.

Features

8MHz MC68008
128K/512K RAM
57600 N82 bit-bang serial port
Serial bootstrap
ATF22V10 as glue logic and bootstrap ROM
64-byte ROM in ATF22V10

Theory of Operation

Refer to here for a short explaination for embedding ROM in 22V10

At power up, 22V10-based ROM provides the program for 68008 until location $40 is access which will switch out the ROM and replaced with RAM. While ROM is enabled, the RAM is enabled and writable so the role of ROM bootstrap is to decode incoming serial data and write to itself starting from location $0. When location $40 is written, the ROM is replaced with RAM but the bootstrap program remained the same. When 512 bytes of serial data are received, the program starts at location $40.

SerRx    equ $fffffff4
page     equ $fffffffe
         org $0
;serial bootstrap with 22V10
;serial receive is hooked up to D[7]
;CPU clock is 8MHz
         dc.l $8000
         dc.l start
         org $14
start:
         lea $0,a0                 ;start serial load from $0
         move.w #$200,d3            ;write 512 bytes into RAM
setup:
         moveq #7,d1               ;8 bits in serial receive
startBit:
         move.b SerRx,d2
         bmi startBit
         movem d0-d7/a0-a3,(a7)     ;wait 1.5 bit time
getSer:
         asl SerRx                  ; shift into extend bit
         roxr.b #1,d2                 ; D2 holds the received value
         movem d0-d7,(a7)           ;wait 1 bit time
         dbra d1,getSer
         move.b d2,(a0)+
         dbra d3,setup

         bra.s bootend               ;prefetch this instruction
         nop
bootend:
;this is location $40
;loaded program resume execution here.