Synchronous Serial Interface bus driver		---------------------------------------   EEEEE X   X PPPP  EEEEE RRRR  IIIII M   M EEEEE N   N TTTTT  AAA  L   E      X X  P   P E     R   R   I   MM MM E     NN  N   T   A   A L   EEEE    X   PPPP  EEEE  RRRR    I   M M M EEEE  N N N   T   AAAAA L   E      X X  P     E     R R     I   M   M E     N  NN   T   A   A L   EEEEE X   X P     EEEEE R  R  IIIII M   M EEEEE N   N   T   A   A LLLLLThis directory holds the SSI bus drivers.  Basically, a SSI bus consistsof the following signals:	stxd	Transmit data	srxd	Receive data	sclk	Clock	sfrm	Frame		Chip selects (1 - n)There may be more than one device on a SSI bus, and each device canhave different timing requirements.  There are several frame formats:1. Texas Instruments Synchronous Serial Frame format   sclk ____~_~_~_~_~_~_~_~____   sfrm ____~~_________________   stxd ------bn..........b0---   srxd ------bn..........b0---   - data latched in on the falling edge of the clock   - data shifted out on the rising edge of the clock2. Motorola SPI frame format   sclk ______~_~_~_~_~_~_~____   sfrm ~~~~________________~~~   stxd -----bn..........b0----   srxd ----.bn..........b0----   - data latched in on the rising edge of the clock   - data shifted out on the falling edge of the clock, or falling edge     of sfrm3. National Microwire format   sclk ______~_~_~_~_~_~_~_~_~_~_~_~_~_____   sfrm ~~~~_____________________________~~~   stxd -----bn......b0---------------------   srxd -----------------bn..........b0.----   - data latched in on the rising edge of the clock   - data shifted out on the falling edge of the clock   - half duplex, one clock between transmission and receptionTypes of devices----------------The following types of devices can be found on a SSP bus:	Sound chips	Keyboard devices	Touch screen devicesKeyboard--------TX:0. Format: cfglen = 8, framelen = 8, clkpol = 1, clk < 250kHz1. select device2. keyboard responds asserting key_atn3. wait 0.1ms to 5ms4. transmit data byte5. wait >= 150us6. repeat step 4 until all data sent7. deselect device8. keyboard responds de-asserting key_atn9. wait >= 120usRX:0. Format: cfglen = 8, framelen = 8, clkpol = 1, clk < 250kHz1. keyboard asserts key_atn2. select device after 0.1ms < 5ms3. read data byte4. wait 150us5. if key_atn still asserted, goto 36. deselect device7. wait >= 120us