Need help with PASM code

Siri

Please help me with this code.I have a hard time to figure out what what is being done here and what are we trying to accomplish with this code.
I am lost at - test DataPinMask,INA wc.The use of Z & C flags are confusing.

SHIFTIN       mov    DataBitMask,#1            'Create BitMask  i.e - 00000001
              shl    DataBitMask,DataBits      'Set number of Bits i.e - 10000000
              mov    Data,#0                   'Clear Data
_ReadNextBit  or     outa,ClockPinMask         'Set Clock pin HIGH  - Start Clock
              test   DataPinMask,ina     wc    'Load "C" with DataPin value
              muxc   Data,DataBitMask       'Move "C" into Data via DataBitMask position
              andn   outa,ClockPinMask      'Set Clock pin LOW   - End Clock
              shr    DataBitMask,#1      wz 'Move BitMask position right by 1           '     
        if_nz jmp   #_ReadNextBit          'Jump to '_ReadNextBit' if there are more bits

A good explaination with details - apppreciated.

Thanks

Siri

Post Edited (Siri) : 9/18/2008 3:35:32 AM GMT

Paul Baker

Hi Siri,
Can you edit you post and put the code in a code box, or attach the spin file? Your code has lost all its formatting.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

Timmoore

SHIFTIN       mov    DataBitMask,#1            'Create BitMask  i.e - 00000001
              shl    DataBitMask,DataBits      'Set number of Bits i.e - 10000000
              mov    Data,#0                   'Clear Data
_ReadNextBit  or     outa,ClockPinMask         'Set Clock pin HIGH  - Start Clock
              test   DataPinMask,ina     wc    'Load "C" with DataPin value
              muxc   Data,DataBitMask       'Move "C" into Data via DataBitMask position
              andn   outa,ClockPinMask      'Set Clock pin LOW   - End Clock
              shr    DataBitMask,#1      wz 'Move BitMask position right by 1           '     
        if_nz jmp   #_ReadNextBit          'Jump to '_ReadNextBit' if there are more bits

Generally it looks like its reading 8 bits from an I/O port and converting it into a byte, the most significant but of the byte is read first.
Each bit needs to be clocked. The data pin is DataPinMask and the Clock pin is ClockPinMask
The device outputs new data on the clock rising edge.
The first 2 instrcutions are setting up where the data in is going in the byte, its reading the most significant bit first.
instruction3 is clearing the output byte
4 is setting the clock high, the device must output a bit on the rising edge of the clock
5 - test is reading the io ports, and setting the carry bit depeding in DataPinMask i/o port. It will have been set with a 1 in the bit psoition for the port, ie for pin 1 it will have a 1 in bit 1.
6 - muxc is setting a bit in the output byte, selected by DataBitMask if the carry is set. So if the io port was 1, the bit is set.
7 - andn takes the clock low - getting ready for the next bit
8 - shr moves the 1 bit in databitmask right 1, i.e. it starts in bit position 7 for hte first test, then 6, etc. and if the 1 has moved out of position 0, then DataBitMask will be zero, so the zero flag will be set
9 - if the zero flag is not set i.e. DataBitMask still has a 1 somewhere, then jump to get the next bit from the io port.

Siri

Tim,
Now if I understand correctly after the " MOV Data,#0 " instruction the Data bit is "00000000" - cleared to zero
The clock is started and begin reading each byte - then the 'Data input pin' state is read via " INA = 0 or 1 " then the "C" flag is set if parity is odd in other words if " INA=1/high"
Then " muxc Data,DataBitMask " sets the MSB of the Data byte to "1" i.e 10000000
Then sets the clock pin low - to start the next reading by making the clock high -
Then " shr DataBitMask,#1 " - moves the bit position Rt - 7th position and sets " Z " flag to "0 - zero" only if NO position to move- Byte is complete
Then " if_nz jmp #_ReadNextBit " if " z " flag is not set - go and get the next bit

This cycle of events continue untill all 8 bits are received and placed in the "Data byte"

I hope this is correct if not steer me in the right direction.

Thanks for the patience and time to teach a rookie beginner.

Siri

Timmoore

Correct, only correction "The clock is started and begin reading each byte" should be reading each bit