Controlling mining rate on private chain

dabooz

I'm using eth (cpp) and I have a private blockchain setup. When I start the miner it mines a block (with no transactions in it) about 1 every second. In my genesis block I have "difficulty": "0x40000000000". Is there a way to:
a) only mine when a transaction arrives, OR
b) reduce the mining rate to once every 10 seconds or so.

thanks

ConradJohnson

Have you sorted how to control block time?

dabooz

Yes and No. We have a stable private network now, mining about every 15 secs. We can increase and decrease block times as needed. My question was really related to development of smart contracts. It's annoying to have the miner running all the time when you are doing contract development and testing. I found Alethzero very confusing and therefore unusable as a UI for contract development. We are using Solidity and the online tool for testing solidity contracts.
We have also played with Embark, what's intriguing here is that it can stop and start the miner when transactions arrive. I'd like to know if the APIs for this are documented such that anyone can use them?