The long term deflation is normally equal to the economic growth, assuming average constant velocity in the long run.
As such, if activity would diminish, deflation would also diminish. In periods of economic recession, a fixed money supply would give rise to inflation.
The only ESSENTIAL difference between inflation and deflation, is that in inflation, you need a financial sector to help protect your savings from value decrease, a financial sector which takes a margin between borrowing and saving ; while in the case of deflation, you don't need that financial intermediate, as a pile of money "brings his own interest". You can still use financial intermediates, to get MORE than the deflation rate, and they can still lend out at a positive nominal interest, but they cannot take too large margins because otherwise the interest they would give you on your savings would become negative.
The only thing for which deflation is bad, is financial institutions. They live essentially on inflation.
The real interest on a loan, in a deflationary scheme, has a lower bound equal to the deflation rate. That is, if the deflation rate is 2%, any loan will cost AT LEAST 2% in real terms, and probably somewhat more. This lower bound on loans avoids people to invest in low-rentability projects. There wouldn't be any "cheap money" with which bubbles can be blown, and which can be misallocated in huge amounts. So the correction for mis investment would come much earlier and wouldn't be able to be pushed back and back with more and more cheap money until there is a major crisis.
But there's a feedback. If that interest rate would be too high, and would cripple businesses, then growth would diminish, and so would deflation rate. So there is no danger for "permanent depression due to deflation". After all, at zero growth, there wouldn't be any deflation either ! And if there would be a depression, you'd get inflation.