[Coco] FPGA 6809

[Dave Philipsen]

Actually, I stand corrected.  Grant doesn't actually "marry" the buses 
in his project and he explains why on his website:

Similarly, all these CPUs have an 8-bit data bus. For the discrete 
components, the data bus is bidirectional. This could have been 
implemented in VHDL as bidirectional to match. However, the VHDL 
components (including RAM and ROM) tend to have separate incoming and 
outgoing data buses - this makes it easier to interface. Therefore the 
8-bit data bus found on a CPU is represented as two separate data buses 
and are connected to the two data buses on the breadboard.

eg
di => cpuDataIn,
do => cpuDataOut



This code in Multicomp attaches the different peripherals to the CPU 
read data bus when the respective chip select goes low:

cpuDataIn <=
interface1DataOut when n_interface1CS = '0' else
interface2DataOut when n_interface2CS = '0' else
sdCardDataOut when n_sdCardCS = '0' else
basRomData when n_basRomCS = '0' else
internalRam1DataOut when n_internalRam1CS= '0' else
sramData when n_externalRamCS= '0' else
x"FF";


An example of connecting the CPU write data bus to an external RAM chip 
is found here:

sramAddress(15 downto 0) <= cpuAddress(15 downto 0);
sramData <= cpuDataOut when n_WR='0' else (others => 'Z');
n_sRamWE <= n_memWR;
n_sRamOE <= n_memRD;
n_sRamCS <= n_externalRamCS;

This is the way he keeps it flexible in order to work with both the 
"internal" VHDL devices which may use separate data buses as well as the 
real external devices that normally use a single bi-directional bus.  
Additionally, there are some physical devices which do not require the 
chip select to be active in order to perform a write operation.

Dave Philipsen




On 8/30/2015 10:42 PM, Bill Nobel wrote:
> I have gone over Grants code extensively and I never found a symbol file for Chip select. I see the CS lines defined, but not such as Roger’s define for a symbol file.
>
> Bill Nobel
>
>> On Aug 30, 2015, at 9:23 PM, Dave Philipsen <dave at davebiz.com> wrote:
>>
>> If you take a look at the VHDL code in Grant Searle's Multicomp project you'll find an example of this too.
>>
>> Dave Philipsen
>>
>>
>> On 8/30/2015 8:53 PM, Bill Nobel wrote:
>>> I have to post this here,  A HUGE shout out to Roger Taylor for directing me in the right direction for the bus marriage on the FPGA versions of the 6809.  I am a noob when it comes to these things and was having trouble marrying the data_in data_out busses together to form a actual bi-directional bus.  I thought I was needing to build a circuit with latches/buffers, and he decides to give me a ~20 line piece of vhdl code that does the whole works based on Chip selects. Woo hoo….
>>>
>>> Bill Nobel
>>>
>>>
>>
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