hi,

I do not wish to start a flame here, just some answers for questions you posted:

OK, but where is a problem with SPI clock?

The problem is that you said about SLAVE mode that this mode may accept clock duration up to 1 machine cycle (1us at 12MHz clock). So if somebody will try to use SPI in slave mode with such frequency (not for vocoder but in another project) then are you need to be claimed for these words?

Original poster needs only 19200 baud to get words from vocoder!

Indeed. Have you known the OCS frequence which original poster works with? I do not know but I have read that he said in one of posts above: "i am wory about missing data if use fosc at 230400 hz."

Btw, I referred to the AT89LS53 chip, why we discuss about AT89C51RD2?

Because you said:
you can use any 51 derivative with serial peripheral interface (SPI), for instance AT89LS53

So why not about AT89C51RD2 for instance as well?

just take this channel transmit data strobe from the vocoder, connect it to the external interrupt pin and use 1-0 (or 0-1) transition for synchronization of the first byte of the frame of a vocoder. Definitely I don't understand what are you talking about.

I try explain what I meant. Sure,we may use transition (with one note - /INTx works with falling edge). As I understand, your idea is wait till strobe (by interrupt or by polling) and then switch SPI macro on. But my question is: which SPI mode should be used?

There are four SPI modes from 0 to 3. Because TX data from vocoder is shifted out with rising edge of SCK so SLAVE should lock them at falling edge when they are stabilized. So we may select SPI mode 1 or 2.
Mode 1 requires /SS pin to be toggled high and then low between each byte transmitted for correct work.
In mode 2, Slave requires one extra clock as start transmission signal.

As for me, so I would preffer to use mode 2 but only if it is possible to run SPI with rising edge of CHAN_TX_STRB signal. For it we need either connect inverter before /INTx or use polling method or utilize derivative which supports positive edge interrupt (an input of PCA of AT89C51RD2 for instance).

Regards,
Oleg