The Direct Answer: Decimal 84 to BCD Conversion
The Binary Coded Decimal (BCD) conversion for the decimal value 84 is 1000 0100 (or 0x84 in packed BCD hexadecimal). Unlike pure binary, where 84 would be 0101 0100, BCD isolates each decimal digit into its own 4-bit nibble. The tens digit (8) becomes 1000, and the ones digit (4) becomes 0100.
BCD encodes each base-10 digit ($d$) using the 8-4-2-1 binary weighting system.
$\text{BCD}(D) = \text{BCD}(d_{tens}) \parallel \text{BCD}(d_{ones})$
For 84: $\text{BCD}(8) = 1000$ and $\text{BCD}(4) = 0100$.
Concatenated: 10000100.
Neighboring Values Reference Table (±20% Range)
When debugging digital logic on an oscilloscope or logic analyzer, you rarely need just one value. Below is a reference table covering the ±20% range around our target value of 84 (spanning 68 to 100). Notice how the lower nibble resets to 0000 every time the upper nibble increments, which is the defining hallmark of BCD versus pure binary.
| Decimal Value | BCD (Binary Nibbles) | Packed BCD (Hex) | Pure Binary (For Contrast) |
|---|---|---|---|
| 68 | 0110 1000 | 0x68 | 0100 0100 |
| 72 | 0111 0010 | 0x72 | 0100 1000 |
| 76 | 0111 0110 | 0x76 | 0100 1100 |
| 80 | 1000 0000 | 0x80 | 0101 0000 |
| 84 | 1000 0100 | 0x84 | 0101 0100 |
| 88 | 1000 1000 | 0x88 | 0101 1000 |
| 92 | 1001 0010 | 0x92 | 0101 1100 |
| 96 | 1001 0110 | 0x96 | 0110 0000 |
| 100 | 0001 0000 0000 | 0x100 | 0110 0100 |
What Fixes the Answer and When Context Shifts
The assumption that fixes the answer above is the use of standard 8-4-2-1 weighted BCD. If your system uses Excess-3 (Stibitz code) or Gray code, the binary output shifts entirely (e.g., in Excess-3, decimal 84 is encoded by adding 3 to each digit first: 11 and 7, yielding 1011 0111). Always verify your encoding scheme against the datasheet of your receiving microcontroller or display driver.
While AC voltage is irrelevant, logic voltage dictates your physical IC selection. A 5V system requires different silicon than a 3.3V or 15V system. Furthermore, the conversion becomes practically meaningless if your input data is already formatted as ASCII hex or if you are performing heavy mathematical operations (where pure binary is vastly more efficient than BCD). BCD is reserved almost exclusively for human-readable decimal output interfaces, like digital panel meters or digital clocks.
Decision Tree: Picking Your BCD Converter IC
Do not waste time writing software translation routines if a hardware state machine can do it in nanoseconds. Use this decision path to select the exact BCD converter IC for your bench or production build.
| Application Requirement | Logic Voltage | Recommended IC Family | Exact Part Number |
|---|---|---|---|
| Decode BCD to drive a 7-segment LED display (Common Cathode) | 2.0V to 6.0V (3.3V or 5V) | 74HC CMOS | MC74HC4511NG (ON Semi) |
| Decode BCD to drive a 7-segment LED display (Common Cathode) | 3.0V to 15.0V (9V or 12V systems) | 4000B CMOS | CD4511BE (Texas Instruments) |
| Convert pure binary input into BCD output | 4.5V to 5.5V (Strict 5V TTL) | 74LS TTL | SN74LS184N (Texas Instruments) |
| Decode BCD into 1-of-10 individual decimal output lines | 3.0V to 15.0V | 4000B CMOS | CD4028BE (Texas Instruments) |
The Concrete Pick: For 90% of modern hobbyist and industrial digital meter projects running on 5V or 3.3V microcontrollers (like an Arduino Uno or ESP32), terminate your search and buy the ON Semiconductor MC74HC4511NG. It features internal lamp-test and blanking pins, sinks up to 25mA per segment (enough to drive standard 7-segment displays without external transistors), and costs roughly $0.65 in single quantities.
Frequently Asked Questions
Why does my 74HC4511 BCD converter display a blank screen when I input '1010'?
Because '1010' (decimal 10) is an invalid BCD state. Standard BCD only utilizes states 0000 through 1001 (0-9). The 74HC4511 is designed to automatically blank the display (turn off all segments) when it detects any input from 1010 to 1111. This is a feature, not a bug, used to suppress leading zeros on multi-digit displays.
Can I use a 74LS184 binary-to-BCD converter with a 3.3V ESP32?
No. The 74LS (Low-power Schottky) family requires a strict 5V supply and recognizes logic HIGH at a minimum of 2.0V, but its output thresholds may not reliably trigger 3.3V CMOS inputs. If you must convert pure binary to BCD in a 3.3V environment, use a 74HC-series equivalent or handle the math in the ESP32's firmware using the snprintf(buffer, sizeof(buffer), "%d", value) function, which is virtually free on a 240MHz dual-core processor.
What is the purpose of the Lamp Test (LT) and Blanking (BL) pins?
On the MC74HC4511, pulling the LT pin LOW forces all seven segments to illuminate, allowing you to verify dead LEDs before troubleshooting the logic circuit. Pulling the BL pin LOW overrides all inputs and blanks the display. The LE (Latch Enable) pin freezes the displayed value even if the BCD input lines change, which is critical when multiplexing multiple displays on a shared data bus.
For further reading on digital logic encoding schemes, refer to the Binary Coded Decimal tutorial on Electronics Tutorials. Always consult the specific MC74HC4511 datasheet from ON Semiconductor or the CD4028B datasheet from Texas Instruments for exact timing diagrams and absolute maximum ratings before soldering your final board.
