Embedded C Cheat Sheet

Large data sheets make it easy to forget the fundamentals of C programming. Enjoy KasperAero's single-page quick reference for embedded C development on the ATtiny1616 and related AVR tinyAVR 0/1/2-series microcontrollers.

It covers essential data types, preprocessor rules, GPIO port and pin control, bitwise operations, TWI/I²C master communication sequences, TMAG3001 Hall-effect sensor register map and mT conversion, common rookie mistakes, and MPLAB X live debug tips.

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Embedded C Cheat Sheet

AVR tinyAVR 0/1/2-Series · ATtiny1616 · MPLAB X · XC8 Compiler

Types & Essential Includes

Type	Meaning
uint8_t	Unsigned 8-bit (0 – 255)
int8_t	Signed 8-bit (–128 to 127)
uint16_t	Unsigned 16-bit (0 – 65535)
int16_t	Signed 16-bit (–32768 to 32767)
uint32_t	Unsigned 32-bit (0 – 4 billion)
bool	true / false — needs <stdbool.h>
float	32-bit floating point
void	No return type / no arguments
static	Variable persists between function calls
volatile	Tells compiler: value may change externally
const	Value cannot be changed after initialization

// Required headers — order matters for F_CPU

#define F_CPU  20000000UL   // MUST come before delay.h

#include <avr/io.h>       // Port/register names

#include <util/delay.h>   // _delay_ms(), _delay_us()

#include <stdint.h>       // uint8_t, int16_t, etc.

#include <stdbool.h>      // bool, true, false

Preprocessor — Text Substitution, Not Code

Runs before compilation. No memory is allocated, no instructions generated. Every occurrence is replaced with the literal value.

#define F_CPU    20000000UL    // 20 MHz clock

#define LED_PIN  PIN3_bm       // bit mask for pin 3

#define THRESH   1000          // magic number → named constant

// Macros with arguments

#define ABS(x)   ((x) < 0 ? -(x) : (x))

#define MAX(a,b) ((a) > (b) ? (a) : (b))

GPIO — Port A, B, C Pin Control

Register / Macro	Action
PORTx.DIRSET = mask	Set pin(s) as OUTPUT
PORTx.DIRCLR = mask	Set pin(s) as INPUT
PORTx.OUTSET = mask	Drive pin(s) HIGH
PORTx.OUTCLR = mask	Drive pin(s) LOW
PORTx.OUTTGL = mask	Toggle pin(s)
PORTx.IN	Read actual pin state (use for inputs)
PORTx.PINnCTRL	Per-pin config: pull-up, invert, etc.
PORT_PULLUPEN_bm	Enable internal pull-up resistor

PIN naming — critical concept

PIN0_bm is just the number 0x01. It carries no port information. The PORT register you write to is what selects the physical pin.

// Same mask, three completely different pins:

PORTB.OUTSET = PIN0_bm;  // → PB0

PORTA.OUTSET = PIN0_bm;  // → PA0  (different!)

PORTC.OUTSET = PIN0_bm;  // → PC0  (different again)

// Best practice: name both port AND pin together

#define LED_PORT  PORTB

#define LED_PIN   PIN0_bm

LED_PORT.OUTSET = LED_PIN;  // unambiguous — always PB0

// Enable pull-up on an input pin

PORTA.PIN1CTRL |= PORT_PULLUPEN_bm;

Bitwise Operations

Op	Purpose
&	AND — mask / read specific bits
\|	OR — set bits without touching others
^	XOR — toggle specific bits
~	NOT — invert all bits
<<	Shift left (multiply by 2 per shift)
>>	Shift right (divide by 2 per shift)
&=	AND-assign — clear bits safely
\|=	OR-assign — set bits safely
^=	XOR-assign — toggle bits safely

// Set bit 4

REG |= (1 << 4);

// Clear bit 4

REG &= ~(1 << 4);

// Toggle bit 4

REG ^= (1 << 4);

// Test if bit 4 is set

if (REG & (1 << 4)) { ... }

// Chained error check — ok stays true only if ALL calls succeed

bool ok = true;

ok &= write_reg(addr, REG1, 0x40);

ok &= write_reg(addr, REG2, 0x02);

if (!ok) { error_blink(); }

TWI0 (I²C Master) — ATtiny1616

Register	Purpose
TWI0.MBAUD	Set clock speed. 95 → ~100 kHz @ 20 MHz
TWI0.MCTRLA	Enable master: TWI_ENABLE_bm
TWI0.MSTATUS	Status flags — check after every byte
TWI0.MADDR	Write device_addr << 1 \| 0(W) or 1(R)
TWI0.MDATA	Write data byte OR read received byte
TWI0.MCTRLB	Send ACK / NACK / STOP commands

Status flags — check after every byte

TWI_WIF_bm	Write complete — byte was sent
TWI_RIF_bm	Read complete — byte received
TWI_RXACK_bm	Device NACKed (1 = NACK = bad)
TWI_ARBLOST_bm	Arbitration lost
TWI_BUSERR_bm	Bus error — invalid START/STOP
TWI_BUSSTATE_IDLE_gc	Force bus idle after init (errata fix)

BAUD formula

// MBAUD = (F_CPU / F_SCL − 10) / 2

// For 100 kHz @ 20 MHz: (200 − 10) / 2 = 95

TWI0.MBAUD   = 95;

TWI0.MCTRLA  = TWI_ENABLE_bm;

TWI0.MSTATUS = TWI_BUSSTATE_IDLE_gc;  // errata workaround

Write one register

// START → addr+W → reg → data → STOP

TWI0.MADDR = (dev_addr << 1) | 0x00;

while (!(TWI0.MSTATUS & TWI_WIF_bm));

if (TWI0.MSTATUS & TWI_RXACK_bm) return false;  // NACK

TWI0.MDATA = reg_addr;

while (!(TWI0.MSTATUS & TWI_WIF_bm));

TWI0.MDATA = data;

while (!(TWI0.MSTATUS & TWI_WIF_bm));

TWI0.MCTRLB = TWI_MCMD_STOP_gc;

Read registers (repeated START)

// Phase 1: set register pointer (write phase)

TWI0.MADDR = (dev_addr << 1) | 0x00;

TWI0.MDATA = reg_addr;

while (!(TWI0.MSTATUS & TWI_WIF_bm));

// Phase 2: repeated START → switch to read

TWI0.MADDR = (dev_addr << 1) | 0x01;

while (!(TWI0.MSTATUS & TWI_RIF_bm));

data[i] = TWI0.MDATA;

// ACK middle bytes, NACK+STOP on last byte

TWI0.MCTRLB = TWI_MCMD_RECVTRANS_gc;           // ACK

TWI0.MCTRLB = TWI_ACKACT_bm | TWI_MCMD_STOP_gc; // NACK+STOP

TMAG3001 Hall Sensor Quick Reference

I²C addresses — set by ADDR pin on PCB

ADDR pin	7-bit Addr	Write byte	Read byte
GND	0x34	0x68	0x69
VCC	0x35	0x6A	0x6B
SDA	0x36	0x6C	0x6D
SCL	0x37	0x6E	0x6F

Key result registers (Table 7-1)

0x12 / 0x13	X axis MSB / LSB
0x14 / 0x15	Y axis MSB / LSB
0x16 / 0x17	Z axis MSB / LSB
0x10 / 0x11	Temperature MSB / LSB
0x18	Conv_Status — conversion complete flag

SENSOR_CONFIG_1 (0x02) — MAG_CH_EN bits [7:4]

0x00	All channels OFF (default)
0x10	X only
0x20	Y only
0x40	Z only
0x70	X, Y, Z all enabled

Convert raw Z reading to milliTesla

// 16-bit signed 2's complement, full scale = 32767

// Change 40.0f to match sensor variant:

//   TMAG3001A1: 40 or 80 mT

//   TMAG3001A2: 120 or 240 mT

float z_mT = ((float)z_raw / 32767.0f) * 40.0f;

MPLAB X — Monitoring Live Data

Where to look	What you get
Window → Debugging → Variables	Auto-shows all locals. Right-click → Decimal
Window → Debugging → Watches	Add any variable or custom expression
Watch expression	(float)z1 / 32767.0 * 40.0 → live mT
Tools → Data Visualizer	Scrolling live graph via UART output
Debug → Pause / F7 / F8	Pause · Step Into · Step Over

PICkit 5 power: Project Properties → PICkit 5 → Power target from PICkit 5. Max 150 mA. Set voltage to 3.3V for TMAG3001 (max VCC = 3.6V — 5V will damage it).

Common Gotchas ⚠

Mistake	Why It Hurts
F_CPU mismatch	delay() wrong timing if F_CPU ≠ actual clock
Wrong PORT for PIN mask	PIN0_bm on PORTA ≠ PIN0_bm on PORTB
Read OUT instead of IN	OUT shows what you drove, not actual pin level
No pull-up on input	Floating pin → random reads, unpredictable behavior
Ignoring TWI NACK	Silently continues; reads garbage data
VCC > 3.6V on TMAG3001	Will permanently damage the sensor
UPDI not disabled after flash	Oscillator stays on; power consumption wrong
Non-atomic register writes	Use \|= and &= not = on shared registers
delay() inside ISR	Never call _delay_ms inside an interrupt handler
Signed vs unsigned compare	int8_t vs uint8_t comparison gives wrong result

KasperAero LLC ATtiny1616 · MPLAB X · XC8