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Counter

Problem: make a hexadecimal counter that displays on the upper 4 digits of a 6 digit 7 segment display.

The display is controlled by two ports:

  • Port 1 (SCAN) that turns on one or more of the digits. Bit 0 turns on the rightmost digit. Bit 5 turns on the leftmost digit. One additional complication: we need to keep the 6th bit of Port 1 high at all times.
  • Port 2 (DISPLAY) turns on and off segments and the decimal point of the currently active display(s).

The numbers 0 - F are represented in 7 segments by the following table:

DB $EB $28 $CD $AD $2E $A7 $E7 $29 $EF $2F $6F $E6 $C3 $EC $C7 $47

The displays need to be scanned rapidly to prevent the perception of flickering. Every second (approximately) the counter is incremented

Byte array

The first step is to declare a nibble-to-7segments table as a one dimensional byte array

\[#EB #28 #CD #AD #2E #A7 #E7 #29 #EF #2F #6F #E6 #C3 #EC #C7 #47]' c!
  • \[ indicates that the numbers following are byte values which will be stored in a byte array allocated on the heap.
  • #EB is an example of a hexadecimal byte value.
  • ] indicates the end of the array. This pushes the address of the array on the stack followed by its length.
  • ' we don't need the length so we drop it.
  • c! we store the 16-bit address of the array in the variable c so we can access it later.

Definition A: convert a nibble to 7 segment display representation

value -- DISPLAY

Write a definition which takes a value in the lower 4 bits 0 - F and converts it to 7 segment display representation

:A #0F& c@+ \@;

Where:

  • :A declare a definition called A
  • #0F& bitwise-AND the top of the stack with the hexadecimal value 0F, this mask everything except the bottom 4 bits
  • c@+ get the address of the byte array and add it to the masked nibble value, this is the address of the 7 segment value
  • \@ read a byte from the address
  • ; end of definition

Definition B: output a nibble to an active segment

Write a definition which takes a 16-bit value and an 8-bit value representing the currently active digit. We are only interested in the lowest 4 bit of the value. The digit is selected by a 1 in Bit 5 to Bit 0. Bit 6 is kept at 1 at all times.

number scan --

:B $ A 2\O #40 | 1\O 10() #40 1\O;

Where:

  • :B declare a definition called B
  • $ swap number with scan
  • A convert the lower 4 bits of number into 7 segment representation
  • 2\O write the 7 segments data out to Port 2 (DISPLAY)
  • #40 | bitwise-OR scan value with hex 40 to keep bit 6 high
  • 1\O write digit selector value to Port 1 (SCAN)
  • 10() delay for about half a millisecond
  • #40 output all 0s to the digits but bit 6 kept high
  • 1\O write digit selector value to Port 1 (SCAN)
  • ; end of definition

Definition C: select next digit, shift nibble down

Write a definition which takes a number and a byte that selects the currently active digit. Replace it with the number shifted right by 4 bits and digit selector bit shifted one bit to the left

number scan -- number' scan'

:C { $ }}}} $;

Where:

  • :C declare a definition called C
  • { shift scan to one bit to left
  • $ swap number to top of stack
  • }}}} shift number one nibble right
  • $ swap new scan to top of stack
  • ; end of definition

Definition E: scan number to display

Take a 16-bit number and display it on the upper 4 7-segment displays.

number --

:E #04 4(%%BC)'' ;

Where:

  • :E declare a definition called E
  • #04 push the first digit to scan, 4 is the third-last digit
  • 4( start a loop which will iterate 4 times
  • %% duplicate the top two stack items
  • B output the lowest 4 bits of number to active segment
  • C select next digit, shift nibble down
  • ) end loop
  • '' drop the top two stack items
  • ; end of definition

Definition F: scan number to display

Take a 16-bit number and display it on the upper 4 7-segment displays.

number --

:F 100(" E)';

Where:

  • :F declare a definition called F
  • 100( loop 100 times
  • " duplicate number
  • E scan number to display
  • ) end loop
  • ' drop number
  • ; end of definition

Count and display

This is the entry point of the program

Count up from zero displaying the value of the loop variable for a second on each iteration.

To run type:

#FFFF( \i@ F ) 0 0B

Where:

  • #FFFF( loop FFFF times
  • \i@ read the value of loop variable i
  • F scan the value of i to the displays
  • ) end of loop
  • 0 0B turn off Ports 1 & 2 but keeping bit 6 of Port 1 high