Electronic – Hand-made interface to SWD to write flash of ARM chip

I'm interested in creating simple interface to SWD to be able to write flash memory of ARM chips. I.e. to create a make-shift programming device – more for sake of exercise.

I started reading the documentation and searching for examples (for I think people already tried this). Currently I have a few questions, for which I would be thankful for guidance:

• I'm going to configure SWDCLK as normal output, and SWDIO as an open-drain (acting as input when it is in high-Z, see below) – I suspect that it is also open-drain on the side of the target device, so with the help of pull-up I will never have a short-circuit by mistake – is this correct?
• is there minimum allowed clock period? or anything will do, even just a few Hertz? Or perhaps this is dependent on target device?
• is it correct that in waiting mode both lines are driven HIGH on the host side, and when the communication begins, the first edge on SWDCLK is falling?
• HOST should change SWDIO on falling edge of SWCLK – and sample data from target (when needed) on falling edge of SWCLK either – right?

I think currently that's enough (though I would be also thankful for short sample of commands sequence to read / write the flash – but I believe I'll find it out anyway soon) – thank you in advance!

Currently I'm trying to use Arduino to create a basic prototype, though I can switch to plain Atmega or LPC111x with FTDI cable at any time… Though I believe this is not important.

P.S. to explain better about SWDIO – with AVR's for example I put the pin to LOW state and then if it is switched to output, it produces 0. However if it is switched to input, it is in high-Z state and really works as input. I.e. I manipulate not PORTx bit, but DDRx instead (while PORTx is permanent 0 and PINx is used for reading).