CompactFlash writes without CE1

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Hello!

I have a problem with my CompactFlash device, based on the SH7727. The
card has its dedicated CE1 and CE2 pins on this device (a built-in PCMCIA
controller is present). However, even when I write to some other memory
area (like UART, which is mapped into a different CE area but has its own
bus), the card is also written to (which, of course, confuses the hell out
of it).

The card says it's "Hitachi XXM2.3.0", "Rev. 3.00" - I read it through the
JTAG connector in the same device. Strangely enough, JTAG-controlled bus
writes to other devices have no effect, so there are no shorts between
different chip enables. Only when the CPU runs (and at a fairly slow bus
frequency at that - 6 MHz), the problem becomes apparent.

More, when I looked at the CE1 line (at the card, not at the processor)
with a scope, I saw *nothing* during the invalid writes. Attaching a small
(1 nF) cap to the line also didn't help (I hoped it would arrest spurious
pulses). The card definitely has proper supply decoupling (100 nF in
parallel with 1 uF ceramic, 47 uF organic tantalum somewhat farther on the
board).

The effect is extremely reliable and repeatable. Even a single write to
the selected address (any address with 0x200 at the end that also asserts
WE) causes a mis-write to the CF card config register at 0x200.

Do you perhaps have any clues? If so, please help.

Baffled,

Stanislaw Skowronek

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