• GPIO1 = I2C_SDA
• GPIO2 = I2C_SCL
• GPIO4 = SR_SER
• GPIO5 = SR_CLK
• GPIO6 = SR_LATCH
• GPIO7 = SR_OE_N
• GPIO8/9/10 = MUX_S0/S1/S2
• GPIO11 = MUX_EN_N
• GPIO3 is strapping/JTAG-related and remains NC. GPIO12, GPIO13, TX and RX remain unloaded spares.
• SR_CLR_N is hardware-controlled by its pull-down and is not assigned to a module GPIO.
• Module 3V3 is test/output only and is not tied to the TLV75533 3V3 rail.

Cpp
#include <Arduino.h>
#include <Wire.h>

#define PIN_I2C_SDA   1
#define PIN_I2C_SCL   2
#define PIN_SR_SER    4
#define PIN_SR_CLK    5
#define PIN_SR_LATCH  6
#define PIN_SR_OE_N   7
#define PIN_MUX_S0    8
#define PIN_MUX_S1    9
#define PIN_MUX_S2    10
#define PIN_MUX_EN_N  11

static bool oneHotOrZero(uint8_t m) { return m == 0 || (m & (m - 1)) == 0; }
static void latch(uint8_t m) {
  shiftOut(PIN_SR_SER, PIN_SR_CLK, MSBFIRST, m);
  digitalWrite(PIN_SR_LATCH, HIGH); digitalWrite(PIN_SR_LATCH, LOW);
}
static void safeInit() {
  pinMode(PIN_SR_OE_N, OUTPUT); digitalWrite(PIN_SR_OE_N, HIGH);
  pinMode(PIN_SR_SER, OUTPUT); digitalWrite(PIN_SR_SER, LOW);
  pinMode(PIN_SR_CLK, OUTPUT); digitalWrite(PIN_SR_CLK, LOW);
  pinMode(PIN_SR_LATCH, OUTPUT); digitalWrite(PIN_SR_LATCH, LOW);
  pinMode(PIN_MUX_EN_N, OUTPUT); digitalWrite(PIN_MUX_EN_N, HIGH);
  pinMode(PIN_MUX_S0, OUTPUT); pinMode(PIN_MUX_S1, OUTPUT); pinMode(PIN_MUX_S2, OUTPUT);
  digitalWrite(PIN_MUX_S0, LOW); digitalWrite(PIN_MUX_S1, LOW); digitalWrite(PIN_MUX_S2, LOW);
  latch(0x00);
  Wire.begin(PIN_I2C_SDA, PIN_I2C_SCL);
}
static bool writeRelayMask(uint8_t mask, bool safetyOK) {
  if (!safetyOK || !oneHotOrZero(mask) || (mask & 0xC0)) return false;
  digitalWrite(PIN_SR_OE_N, HIGH);
  latch(0x00);
  delay(250); // verify/tune against relay release time
  latch(mask);
  digitalWrite(PIN_SR_OE_N, LOW);
  return true;
}
void setup() { safeInit(); }
void loop() { delay(1000); }