🚀 go-pugleaf

In article <1992Apr7.183518.1356@ucsu.Colorado.EDU> rainer@spot.Colorado.EDU (Rainer Malzbender) writes:

>Basically what you need is a signal that says "I'm booting". If you're
>building a system with a UART, a good choice is the 2681. In addition
>to two uarts, it also has several I/O pins which can be used for any
>purpose whatever. I used one of these as my "boot" flag - it gets set to
>some state (I forget which - high, I think) on power up, and my software
>resets it to the other state after we're on the air.
>
>This boot signal goes into the address decode PAL, which generates device
>selects based on the address and a few other signals. The PAL maps ROM
>into low memory if the boot flag is on, high memory otherwise.

However, this may complicate things if you intend to use the 68k's
RESET instruction.  This momentarily turn's the chip's RESET/ pin into
an output and pulls it TRUE, resetting any peripherals connected to
it.  However, the RESET instruction does not reset the 68k.  This means
that your program must clear the "boot" flag after a RESET instruction
before it attempts to access RAM.

This all assumes that you have designed your hardware properly: it is
tempting to tie HALT/ and RESET/ together to the output of whatever
generates the power-on reset signal.  If you do this, the RESET
instruction will pull HALT/ TRUE, causing a reset of the 68k in
addition to all of the peripherals.

-- 
Roger Chaplin / Instruments Division Engineering
chaplin@keinstr.uucp / CI$: 76307,3506
nuthin fancy here . . .