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PostPosted: Mon May 27, 2024 5:58 pm 
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How well does SRAM cope with floating address pins when it's enabled (/CE, CE2) but neither outputting (/OE) nor storing (/WE)?

I am designing a read/write-round-robin-algorithm for SRAM in a VGA circuit and was thinking about how strict I need to be about always having some kind of data on the address bus.


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PostPosted: Mon May 27, 2024 6:09 pm 
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If CMOS inputs are left floating, power-supply current may increase a lot.  They go to a totem pole arrangement of MOS transistors, and the one and the top should be totally off while the one at the bottom is on, and vice-versa.  If the input is not connected, they could both be at least partially on.  However, if there's nothing driving the line in either direction and all loads are CMOS, the capacitance will hold the last driven voltage a surprisingly long time, not just nanoseconds, or even microseconds, but milliseconds.

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PostPosted: Mon May 27, 2024 6:31 pm 
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I forgot to mention, I am not talking about the 6502 bus, as it's a completely separate design that I want to interface using address and data-latches, in a first incarnation connected to the 6522 in some way, then if successful, later on integrated in the SBC (with the latches connected using memory mapped I/O).

What I have in mind is to have something like a 74AHC138 cycle through the lowest 3 bits of a column value, and one of the '138 outputs will trigger a write to RAM from the above mentioned latches, and 2-4 of the other will do various reads from memory for a simple state machine to form the next 8 pixels to be displayed. Kind of. Then there might be a few outputs from the '138 left unconnected where nothing is driving the address bus (I don't think I need to do 7 memory accesses per 8 pixels and I am thinking of leaving some "padding" around the write to avoid bus contention).

Sounds like, while not ideal, it will be kind of OK to leave the address bus floating for a few ~80ns periods (a 12.5 MHz pixel clock).

(FWIW the RAM in question is an Alliance Memory AS7C164A-15PCN and I am using all CMOS ICs, some HC, some AHC)


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PostPosted: Mon May 27, 2024 6:55 pm 
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Actually when I think about it, it might be better to let the "write-latch" drive the address bus for half of those 8 steps (say using bit 2 of the x-coordinate as latch output enable instead of an output from the '138) and then put the /WE trigger from the '138 somewhere in the middle of those 4 steps to be sure memory corruption is not an issue. That also leaves less steps to fill and reduces floating bus time that could possibly cause some ruckus. Sorry for the sudden rubberducking :D


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