Voltmeter with AT89C2051

• Electrical sheet:
  The hardware is a simple integrator followed by a trigger.
  Both is build with a dual opamp with rail to rail output.
  The trigger output must be connected to the INT0 input.
  
• Initialization:
  Since I use T1 as baud rate generator, T0 is divided in TL0 and TH0 to measure period time and high time. Both timers are cascaded to 3 Byte with software in the timer interrupt routines.
  Measuring is finished with the high to low changing of the trigger output and the next measuring is started.
  The external interrupt must be executed with the highest priority.
  The resolution error is given by the response time of the external interrupt and can be up to 5 cycles (or 3, if no MUL or DIV instruction is used).
  
• Interrupt Routines
  Both Timer interrupts are simple cascaded with DJNZ instructions. On Overflow of the period time measuring an error flag was set and measuring was finished.
  The external interrupt handler is very tricky !
  Both timer registers are read and cleared with XCH instruction, so must not be stopped.
  But the high values can not be read out at this time, since it can be, that an overflow is pending or in progress on one or both timers. To solve this problem, the interrupt priority was inverted and a second external interrupt is forced by setting the IE0 bit. So after the first RETI the timer overflows are handled or completed with highest priority and only after this the external interrupt is executed again. Then the higher values are read and cleared.
  Since the timers are cleared, a timer interrupt of the next measuring is not executed until the next 256 cycles. This is the maximum time to handle all interrupts (include SIO interrupt).
  
• Get Counting values
  The counting values can be read out  if the conversion done flag was set. If it was detect as set after reading, a new measuring was finished and the reading must be repeat (it can contain values mixed from the older and the new measuring).
  The higher bytes are 2's complement, since the DJNZ instruction was used. The result is expand to 4 Bytes. Then you can  calculate the input voltage.