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--- en:multiasm:exercisesbook:avr:sut [2026/04/30 09:57] – [Hardware reference] pczekalski
+++ en:multiasm:exercisesbook:avr:sut [2026/05/04 14:50] (current) – [Visualising Instruction Execution Time Using an Oscilloscope] pczekalski
@@ Line 31: / Line 31: @@
-==== Handling of the buffered 4 digit, 7-segment display ====
+==== Handling of the buffered 4-digit, 7-segment display ====
 To display a digit in the 4x7seg. display, there are two definitions needed: the shape of a digit (or other symbol), and its position (1,2,3,4: a binary mask).
@@ Line 42: / Line 42: @@
     .byte      0xC0, 0xF9, 0xA4, 0xB0, 0x99, 0x92, 0x82, 0xF8, 0x80, 0x90
 </code>
+In a common-anode configuration, the active signal to turn on a segment is LOW (0), and to turn it off, it is HIGH (1). The state of a single digit is represented by an 8-bit mask: 7 segments to build the symbol and a DP (decimal point). For example, a digit 7 is represented by bits corresponding to segments "a", "b", and "c" set to 0 (to turn segments "a", "b", and "c" on) and the remaining bits set to 1 (to turn them off), so the corresponding binary value looks as follows: 11111000b, hence the hexadecimal value is 0xF8 (as in the code above). The MSB bit represents DP, and the LSB segment "a". This definition affects how one loads data into the shift register: starting from MSB towards LSB, because of the way the register is built and connected to the segments - refer to the function ''display_digit'' below.
 <note tip>Naturally, it is possible to expand those definitions to display other symbols, e.g., hexadecimal digits such as A,b,C,d,E,F.</note>
@@ Line 145: / Line 147: @@
 </code>
-Note: registers in this schema store data for only ONE digit. Iterating over digits and displaying them allows it to represent a full, multi-digit number. To display, e.g., 1023, it is necessary to handle each digit separately: "1", "0", "2", and "3", and to repeat this process continuously. If you stop, only the last digit will be visible.\\
+<note important>Note: registers in this schema store data for only ONE digit. Iterating over digits and displaying them allows it to represent a full, multi-digit number. To display, e.g., 1023, it is necessary to handle each digit separately: "1", "0", "2", and "3", and to repeat this process continuously. If you stop, only the last digit will be visible.</note>
-** Display single digit: sample code to display multiple digits in a loop **\\
+<note tip>Changing the definitions of the symbols stored in ''segment_masks'' enables you to easily present characters other than numbers. Think about ''segment_masks'' as a font definition that defines how a symbol looks.</note>
+** Display single digit: how to use it to display a number? **\\
 Sample code that uses the function declared above and displays 1975 is presented below. Note, the MCU runs here at full speed, constantly updating the display. While it is not necessary to (a minimum, comfortable LED display refresh rate should be around 10Hz), we do not present such a solution here for the sake of simplicity. It is common to address timers for this job to periodically refresh the screen.
 <code asm>
@@ Line 186: / Line 190: @@
     clr r25
     clr r23
-    ; --- Idle Main Loop ---
+    ; --- Main Loop, displays in sequence 1->9->7->5 ---
 LOOP:
     ldi r24,0
@@ Line 209: / Line 213: @@
 </code>
 In the function above, we used fixed (constant) digits to display. A common scenario, however, is when the number is stored in some register or in a memory variable.
 ** Convert number to digits: function definition **\\
@@ Line 337: / Line 340: @@
 <note tip>Nodes are interconnected in pairs: 1-2, 3-4, 5-6, 7-8, 9-10. Scenarios for data transmission between MCUs require booking and the use of correct nodes for sending and receiving messages.</note>
+==== Visualising Instruction Execution Time Using an Oscilloscope ====
+Let's try to visualise how code operates the GPIO. Naturally, in the remote lab, it is not possible to do it remotely, so here we present some desk-based experiments.\\
+The ''LAT_PIN'' is GPIO4, and an oscilloscope is connected to it.
+In the function that displays a single digit, there is a section that loads a binary mask into the internal registers, enabling the LED segments that constitute the digit to be turned on and off. It is:
+<code asm>
+...
+    sbi LAT_PORT, LAT_PIN
+    cbi LAT_PORT, LAT_PIN
+...
+</code>
+The figures {{ref>arduinounodigitoscilloscope1}} and {{ref>arduinounodigitoscilloscope2}} present the ''LAT_PIN'' signal, called periodically during the display of the consecutive digits (they represent the same signal but differ by the oscilloscope time base for better observation).\\
+''SBI'' causes the signal to rise, while ''CBI'' to fall. Thus, the HIGH time is the exact time during which the CBI instruction executes. It takes about 120-130ns.\\
+The Arduino Uno operates at 16 MHz, so each cycle is 1/16000000 s, which is about 63 ns. According to the documentation, ''CBI'' takes 2 cycles, which is ~126ns.
+<figure arduinounodigitoscilloscope1>
+{{:en:multiasm:exercisesbook:avr:pic_1054.png?300|}}
+<caption>''LAT_PIN'' signal (50ns time base)</caption>
+</figure>
+<figure arduinounodigitoscilloscope2>
+{{:en:multiasm:exercisesbook:avr:pic_1055.png?300|}}
+<caption>''LAT_PIN'' signal (25ns time base)</caption>
+</figure>

en/multiasm/exercisesbook/avr/sut.1777532267.txt.gz · Last modified: 2026/04/30 09:57 by pczekalski