Wallner, Daniel

PPX16 mcu

After a succesful interrupt, the INTE flag is set back. So no more external Interrupt ist possible anymore. So the INTE status has to be saved and set back to the old status before returning from the inerrupt routine.

External Interrupt doesn't work properly

The FSR is updated in the next clock cycle after the instruction that cahange it.
For example a SUBWF that changes the CARRY updates the FSR in the next cycle after execution (because of the pipelining).
Because of this behavior a BTFSS after a SUBWF (in decrementing loops for example) doesn't work properly.

late FSR update

An ADDWF PCL instruction is not correctly executed. Being a PCL modify instruction (like RETURN, RETFIE, RETLW, CALL, GOTO) the next instruction should be skipped.
The solution is to modify PPC_Ctrl at line 138 bay adding this additional or Inst (13 downto 0) = "00011110000010" --ADDWF PCL

ADDWF PCL operation

RLF instruction does not work correct.

This happens on RLF f, 1

The result should be stored back into register f but it doesn't.

RLF operation

Hi all,

I was experimenting with the PPX16 core and found a bug. I've also 
contacted the author of this core, Daniel Wallner.

The problem I've came across has to do with the Carry flag and the 
SUBLW command. It seems that the carry flag is not set correctly, i'll 
explain:

The instruction description for SUBLW from the PIC16F84 datasheet 
says; When the result is positive or zero the carry flag should be '1';. 
When the result is negative the carry flag should be '0'. 

The PPX16 core is doing the opposite; When the result is positive the 
carry flag is '0', when the result is negative or zero the carry flag is '1'.


Here's a code snippet to give an example:

MOVLW 6
MOVWF main_w_L0
MOVF main_w_L0, 0
SUBLW 5
BTFSS STATUS, C ; Carry flag is set; not correct

MOVLW 5
MOVWF main_w_L0
MOVF main_w_L0, 0
SUBLW 5
BTFSS STATUS, C ; Carry flag is set; correct

MOVLW 4
MOVWF main_w_L0
MOVF main_w_L0, 0
SUBLW 5
BTFSS STATUS, C ; Carry flag is not set; not correct


I've tested this by outputting the current opcode on the seven segment 
display and used a switch to manually create the clock for the core so I 
could single step through the opcodes.


Workaround: Only use the SUBWF command which works correct.

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PPX16 mcu