| ??? 03/28/06 12:15 Read: times |
#113288 - Why using this processor pins left...? Responding to: ???'s previous message |
at this time there are only two persons who understand what I want. In this thread suppuse that the processor is an 8bits(6502).I don't want to build a memory system with a memory chip as suggested Erik, I know that we could build up a memory system with chips equiped of large size but is not my interesting. Before using Decoder circuit and large size memory from only one chip, I want to know the background of this technical skill.
Kai has put a link but do not forget I mentinoed first in my thread " I'm a biginner..." this diagram is hard for me to understand well the decoding "http://www.pjrc.com/tech/midi-drums/sch-cpu.html" Andy are right when he said Andy Neil said:
The "bible" assumes that you are familiar with the basic concepts of
addressing - I don't think Cyrille is there, yet. I'm not familiar with this concept or I have not the backgroung of it so I want to train with it ! Erik Malund said: I have the 8051 data sheet. The problem is simple if we use only one memory chip, but what if we have to use 2,3,4 memories chips? let say we want a memory map of 32k *8bits but we could just find out the 32k *4bits. I draw up the schematic but I don't know to paste it on the thread. for this task I will need 2 chips (maybe 2 RAMs) the number of addresses lines I need from the processor is 5; the numbers of addresses lines from each chip is 5; number of data lines from the CPU data bus is 8; the data lines from each chip is 4; The R/W line of each chip are link together, it is the same for the /CS line. The R/W line would be connected to the R/W line of the processor, but what is about the /CS of the chip ? how do would you build a decoder to select one of both chips?
if we are talking about 8051 or 8031 there is no need to "select either
the ROM or RAM" that is done by !RD and !PSEN To use chips of identical size such as "only 2k x 8" you need an 1 of x decoder, such as a 1 of 4 decoder or 1 of 16 decoder, both are avilable as standard 74xx chips. Anyhow, why even do that, you can buy a 128*8 RAM for $3 and wiring up a lot of 2k RAMs will cost you way more. Zeeshan Ashraf has start with it but is stopped Zeeshan Ashraf said: Ok why the OR guate? how do you reach to the state " I will build my decoder with the OR guate or 74HC138 by using this processor pins...left?
Remember, the OR gate produces a 1 output when all or any of its inputs
are at logic 1. So, the OR gate will give a 0 output when all the upper 3 lines are at logic 0, and since the CS line is supposed to be 0 to enable the RAM IC, this will work. Once again sorry for my bad english which some one dislike. |
