[Coco] 6x09 read and write strobes.

[jdaggett at gate.net]

Phil

Not sure if this is too late or not. 

The 6809 use the E clock to primarily sync with peripherals as well as internal uses. The Q 
clock is an internal clock. The address bus as well as the RW line are all valid on the falling 
edge of the Qclock after the falling edge of the Eclock. The falling edge of the Eclock starts a 
new cycle. You can generate an Intel compatible Write enable output by inverting the RW line 
and NANDing it with the Eclock.  A comparable Output Enable read signal is just NANDing 
the RW line with the E clock. 


james

On 4 Dec 2011 at 15:12, Phill Harvey-Smith wrote:

> Hi all,
> 
> I am currently developing a SD/MMC interface for the Dragon and CoCo, 
> that uses a Xilinx CPLD to interface between the 6809 and the AVR on the 
> board.
> 
> I am however having some problems with random lockups / crashes that 
> don't seem to be software related as I have tested the software in mess 
> and it will run flawless ly there.
> 
> The thing I am unsure about is the read and write strobes and which 
> clocks they should be based upon.
> 
> Currently in my Verilog I have :
> 
> // Generate OE and WE signals, only do this if Reset is high !
> assign RD			  & RW & Reset;
> assign WR			
> assign nRD			
> assign nWR			
> 
> This way RD and WR are positive logic, so I am latiching into the CPLD 
> on the rising edge of WR and outputting to the 6809 bus while RD is 
> true. nRD and nWR are the read and write strobes for the onboard ROM and 
> RAM, the RamWP is in a latched register and allows me to write protect 
> the RAM.
> 
> Is this the correct logic ?
> 
> I have seen (on the DragonDos schematic, the read and write strobes for 
> the WD2797 are similar to the ones above based off both clocks, however 
> the write strobe for the drive/density select latch is based off E.
> 
> Comments anyone ?
> 
> Cheers.
> 
> Phill.
> 
> -- 
> Phill Harvey-Smith, Programmer, Hardware hacker, and general eccentric !
> 
> "You can twist perceptions, but reality won't budge" -- Rush.
> 
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