Atari LETA on a CPLD


Test Mode

TEST provides a pass through for reading the state of the DIR and CLK pins and counter Reset

  • Normal counting based on DIR & CLK
  • A read operation puts counter selected by A0, A1 onto the DQ outputs
  • All counters are rest
  • The inverted state of the DIR and CLK inputs are passed through to the DQ outputs during a read
  • A0/A1 inputs are meaningless, all addresses of the chip return the same result.

On the Rampart board this is used to the read the state of the joystick inputs, for the 2 player version, quite a useful feature.

Corresponding DQ output to  DIR & CLK pins with TEST='1'

DQ0 = inverted DIR0
DQ1 = inverted CLK0
DQ2 = inverted DIR1
DQ3 = inverted CLK1
DQ4 = inverted DIR2 
DQ5 = inverted CLK2
DQ6 = inverted DIR3 
DQ7 = inverted CLK3

Summary of Features

So pulling together the features of the Leta.
  • 4 individually readable internal quadrature counters  
  • simple digital noise filter 
  • 2 resolutions of counting
  • input pass-thu' mode with counter clear
  • simple bus interface with Chip Select & 2 bit Address 



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