Using timer(s) to capture multiple inputs

I got it all working - thanks!

Here is my code in case it's useful to anyone...

var DEFS={
  TIM1: {
    base: 0x40010000,
    pin: A8
  },
  TIM3: {
    base: 0x40000400,
    pin: B4
  },
  TIM4: {
    base: 0x40000800,
    pin: B6
  }
};

function init(tim) {
  const def=DEFS[tim];
  if ( !def ) {
    throw "Invalid timer!";
  }
  const BASE=def.base;
  
  // slave mode control register
  var SMCR = BASE+0x0008;
  // event generation register
  var EGR = BASE+0x0014;
  // Capture compare mode register
  var CCMR1 = BASE+0x0018;
  // Capture/compare enable register
  var CCER = BASE+0x0020;
  // counter
  var CNT = BASE+0x0024;
  // prescaler
  var PSC = BASE+0x0028;
  // auto reload register
  var ARR = BASE+0x002C;

  // enable PWM on A8 (TIM1 CH1)
  analogWrite(def.pin,0.5,{freq:10});
  // CC1E = 0 (Turn channel 1 off)
  poke16(CCER, peek16(CCER) & ~1);
  // CC1S[1:0]=01 (rising edge), IC1F[7:4]=0 (no filter)
  poke16(CCMR1, (peek16(CCMR1) & ~0b11110011) | (0b00000001));
  // CC1P=0, CC1NP=0 (detect rising edge), CC1E[0] = 1 (Turn channel 1 on)
  poke16(CCER, peek16(CCER) & ~(0b1011) | (0b0001));
  // SMS[2:0]=111 (ext clock), TS[6:4]=101 (CH1 as trigger)
  poke16(SMCR, (peek16(SMCR) & ~0b1110111) | 0b1010111);
  // Prescaler to 0 - use every transition
  poke16(PSC, 0);
  // auto-reload with the full range of values
  poke16(ARR, 65535);
  // poke the UG[0] bit to reset the counter and update the prescaler
  poke16(EGR, 1);

  return {
    get: function() {
      return peek16(CNT);
    },
    reset: function() {
      poke16(CNT, 0);
    }
  };
}

const TIM1=init("TIM1");
const TIM4=init("TIM4");
const TIM3=init("TIM3");

function update(){
  g.clear();
  g.drawString("Steps X: "+TIM1.get(), 2, 2);
  g.drawString("Steps Y: "+TIM4.get(), 2, 20);
  g.drawString("Steps Z: "+TIM3.get(), 2, 38);
  g.flip(); 
}

// SPI
var s = new SPI();
s.setup({mosi: B15 /* D1 */, sck:B13 /* D0 */});
var g = require("SH1106").connectSPI(s, B14 /* DC */, B10 /* RST - can leave as undefined */, function() {
  setInterval(update, 100);
});

setWatch(function(e) {
  TIM1.reset();
  TIM4.reset();
  TIM3.reset();
}, BTN, { repeat: true });