[Oberon] Oberon on FPGA - how much does the system "stallx"?

[Jörg]

Hi Davide

The RISC5 is clocked at 25 MHz. Because the VGA driver “steals” some time, the RISC5 effectively runs at ~23.25 MHz.

Jörg

 

From: Davide Della Casa [mailto:davidedc at gmail.com] 
Sent: Montag, 18. Mai 2015 20:35
To: oberon at lists.inf.ethz.ch
Subject: [Oberon] Oberon on FPGA - how much does the system "stallx"?

 

Hi,

 

I'm reading on how Oberon of FPGA manages video generation - the chosen way is to have SRAM in common between CPU and video system.

 

So (reading the docs) the "stallx" line is used to arbitrate: "The signal (wire) dspreq stalls the processor (stallX) and decides whether the memory address (SRAdr) should be taken from the processor (adr) or the display controller (vidadr)".

 

Now I'm wondering what percentage of time does the CPU stall because of this? Is the CPU only effectively free to work during the "invisible lines" and "beam reset" times? Or is there significant time available while "racing the beam"?

 

Also I'm curious to know whether alternate solutions could potentially be in the cards to avoid the contention

 

a) dual port SRAM (too expensive / too many pins needed?)

b) SRAM used in two "blocks" - one dedicated to screen and other so contention is reduced (too many pins needed?) - potentially with a way for the CPU to know if the "video" block is available.

c) ...?

 

(I'd think that such considerations could be worthy of inclusion in the docs by the way)

 

Any links welcome about other typical arrangements of interest that are found in fpga projects "in the wild" in respect to video management...

 

Cheers,

Davide Della Casa

-------------- next part --------------
An HTML attachment was scrubbed...
URL: https://lists.inf.ethz.ch/pipermail/oberon/attachments/20150519/dba73e04/attachment.html 
-------------- next part --------------
A non-text attachment was scrubbed...
Name: smime.p7s
Type: application/x-pkcs7-signature
Size: 4847 bytes
Desc: not available
Url : https://lists.inf.ethz.ch/pipermail/oberon/attachments/20150519/dba73e04/attachment.bin