Thanks for the reply.
I checked out the spi_comm_master sample app.
It starts to send SPI data as master in application_spiffy2_send_bytes called from the following functions.
- spi_comm_master_fine_timeout ... Fine Timer Callback
- application_gpio_interrupt_handler ... GPIO interrupt handler
- spi_comm_master_write_handler ... BLE write handler
I understood that my app must use Fine Timer Callback to send data more than 15 bytes if it does not support FlowControl with slave !