[Coco] GIME Question

[jdaggett at gate.net]

On 11 Jun 2007 at 14:45, Roger Merchberger wrote:

> How much extra load would a "spare ROM" in High-Z (unselected) mode
> add to the address buss?
> 
> Also (second but mainly unrelated question) if the ROM isn't CMOS,
> could you yank the stock ROM and solder in 2 CMOS EPROMs (say,
> 27c256's) and keep within the standard buss load?

Roger

Even deselected the address input pins from the second EPROM will still have an 
effect on the 6809. The input capacitance will not go away just by taking the CE 
line high. This will have an effect on the switching time and thus the average 
current draw during switching. 

>From the specs the MC6809 can source 145 uA through the address pins and 
maintain a voltage of 2.4 VDC when VCC is at 4.5 VDC. A conservative estimate 
is that this is sufficient to drive 7 LSTTL/CMOS loads. This is based on the fact 
that each LSTTL or CMOS load requires 20 uA for input high. The MC6809 can 
sink about 2 mA and maintain an input low voltage of Vss + 0.5 VDC. Again 
conservative estimates are that the MC6809 can drive about 5 LSTTL loads if the 
sink current is 400 uA from each device. CMOS will be more as most 
CMOS/HCMOS will only cause the 6809 to sink about 100uA per device. 

Yes if both ROMS were CMOS then you do lessen the current to some extent. 
Again  you do have differences between input capacitance for the various logic 
families, LSTTL, NMOS, CMOS, and HCMOS to contend with. 

By the way the ROM in most standard Coco3's should be a 23C256, CMOS. That 
is the mask version of the 27C256 CMOS UV erasable EPROM. 


james