Computing stuff tied to the physical world

Memory Plug redux

In Hardware on Nov 15, 2009 at 00:01

Another case of me mixing up specs. Here’s the original prototype:


And here’s the latest one:


Guess what… the original prototype is ok. The “latest” one is bad :(

While testing the original plug, I noticed that multiple chips showed up at I2C addresses 0x50, 0x52, etc – i.e. with a gap in the addressing.

I thought this was a mistake in connecting the E1 and E2 address lines, but it turns out that the lower address bit is needed for 128 Kb addressing (while testing, I used some spare 64 Kbyte units). So without thinking, I changed the lines and had “final” versions made.

But it was no mistake… an M24M01 needs 2 I2C addresses to access both 64 Kbyte banks of its memory:

Screen shot 2009-11-08 at 19.27.04.png

So, in other words the latest plugs can’t hold 4 chips of type M24M01 – only two, in positions 0 and 2.

The original plug works as intended, with each 128 Kbyte chip responding to two I2C addresses. So a fully populated plug looks as eight different I2C memory chips, each capable of storing 64 Kbyte.

Doh. I’ll need to re-order. Until then – the remaining old / green plugs will be shipped.

Note: if you ordered memory plugs and received blue ones, and if you want to be able to use them with more than two 128 Kbyte memory chips, let me know and I’ll send you the boards which properly support all 4 chips.