53775



     (R) Beads the serial port status. It also returns values governed by
     a signal on the digital track of the cassette tape. You can generate
     certain values using the SOUND command in BASIC and a PEEK
     to SKSTAT:

     SOUND 0,5,10,15 returns a value to here of 255 (or, on
       occasion, 127).
     SOUND 0,8,10,3 returns a value of 239.

     This is handy for adding a voice track to Atari tapes. You use the
     left channel for your voice track and the right for the tone(s) you
     want to use as cuing marks. You can use the speaker on your TV to
     generate the tones by placing the right microphone directly in
     front of the speaker. The computer will register these tones in this
     register when it encounters them during a later cassette load. See
     COMPUTE!, July 1981, for some other suggestions on doing this.
     Bemember, you can turn the cassette off by POKEing 54018
     ($D302) with 60 ($3C) and back on with 52 ($34).

     Bits in the SKCTL (W) register are normally zero and perform the
     functions below when set to one. The status when used as (R) is
     listed below the write (W) function:

     Bit   Function
     0     (W) Enable keyboard debounce circuits.
           (R) Not used by SKSTAT.
     1     (W) Enable keyboard scanning circuit.
           (R) Serial input shift register busy.
     2     (W) Fast pot scan: the pot scan counter completes its
           sequence in two TV line times instead of one frame time (228
           scan lines). Not as accurate as the normal pot scan,
           however.
           (R) the last key is still pressed.
     3     (W) Serial output is transmitted as a two-tone signal rather
           than a logic true/false. POKEY two-tone mode.
           (R) The shift key is pressed.
     4,5,6 (W) Serial port mode control used to set the bi-directional
           clock lines so that you can either receive external clock data
           or provide clock data to external devices (see the Hardware
           Manual, p. II.27). There are two pins on the serial port for
           Clock IN and Clock OUT data. See the OS User's Manual,
           p. 133.
     4     (R) Data can be read directly from the serial input port,
           ignoring the shift register.
     5     (R) Keyboard over-run. Reset BITs 7 to 5 (latches) to one
           using SKRES at 53770 ($D20A).
     6     (R) Serial data input over-run. Reset latches as above.
     7     (W) Force break (serial output to zero).