$regfile = "2313def.dat"
$crystal = 4000000

Config Sda = Portd.5
Config Scl = Portd.4
'default I2C speed
Config I2cdelay = 5
'LED on this pin
Config Pind.6 = Output
'Int from PCF8574 to Int0 pin
Config Pind.2 = Input
Config Int0 = Falling
Config Lcd = 16 * 2
Config Lcdpin = Pin , Db4 = Portb.4 , Db5 = Portb.5 , Db6 = Portb.6 , Db7 = Portb.7 , E = Portb.3 , Rs = Portb.2
Config Lcdmode = Port

Const Pcf8574write = &H40
Const Pcf8574read = &H41
'Optical encoder AB is on two low bits
Const Optencmask = &B00000011
'Need to keep the B-bit
Const Optencbmask = &B000000001

Dim Pcf8574input As Byte
Dim Encoderval As Byte
Dim Encounter As Integer
Dim Oldbval As Byte

Encounter = 0
Oldbval = 0
Cls
Lcd "encounter:"

On Int0 Pcfint

'Set PCF8574 to input, masked pins high (pull-up)
I2cstart
I2cwbyte Pcf8574write
I2cwbyte Optencmask
I2cstop

Enable Interrupts
Enable Int0


'Report encoder counter value in an otherwise
'empty main loop.
'Observe LED on Portd.6 : if it goes off or stays on,
'probably too many interrupts arrive
Do
  Set Portd.6
  Waitms 5
  Locate 1 , 12
  Lcd Encounter ; "      "
  Reset Portd.6
  Waitms 5
Loop

'PCF8574 interrupt routine
Pcfint:
  'read the input pins
  I2cstart
  I2cwbyte Pcf8574read
  I2crbyte Pcf8574input , Nack
  I2cstop
  'mask to keep only encoder AB value
  Encoderval = Pcf8574input And Optencmask
  'add old B value if it is 1
  If Oldbval = 1 Then
    Encoderval = Encoderval + 4
  End If
  '<2 or >5 means up, remainder means down
  If Encoderval < 2 Then
    Decr Encounter
  Elseif Encoderval > 5 Then
    Decr Encounter
  Else
    Incr Encounter
  End If
  'keep value of B for next interrupt
  Oldbval = Pcf8574input And Optencbmask
Return

End