firmware: switch the DAC off too when switching off Vio

Without this, the DAC still outputs it's control voltage even if Vio is off.
This goes through the feedback resistors to the output. I measured about
15 mV without this patch on a port that was previously enabled.
With this patch it properly goes down to zero.

While such a low voltage usually shouldn't be a problem, I consider it
unclean design, especially if it is so easy to fix.

firmware/dac_ldo.c

@@ -60,21 +60,23 @@ bool iobuf_set_voltage(uint8_t mask, __xdata const uint16_t *millivolts_ptr) {
   if((mask & IO_BUF_A) && millivolts > glasgow_config.voltage_limit[0]) return false;
   if((mask & IO_BUF_B) && millivolts > glasgow_config.voltage_limit[1]) return false;
 
-  // Just disable the LDOs, DAC power is irrelevant
   if(millivolts == 0) {
+    // disable the LDOs before any I2C comms, we may need to be fast in case of an alert
     IOD &= ~pin_mask;
-    return true;
-  }
-
-  // Compute the DAC code word
-  if(millivolts < MIN_VOLTAGE || millivolts > MAX_VOLTAGE)
-    return false;
 
-  // Offset 1650, slope -15.2, 0x1000/15.2 = 269
-  // The DAC has a 12-bit code word, so we only shift back by 8
-  code_word = (254 << 4) - ((((millivolts - 1650) >> 4) * 269) >> 4);
-  code_bytes[0] = code_word >> 8;
-  code_bytes[1] = code_word & 0xff;
+    code_bytes[0] = 0;
+    code_bytes[1] = 0;
+  } else {
+    // Compute the DAC code word
+    if(millivolts < MIN_VOLTAGE || millivolts > MAX_VOLTAGE)
+      return false;
+
+    // Offset 1650, slope -15.2, 0x1000/15.2 = 269
+    // The DAC has a 12-bit code word, so we only shift back by 8
+    code_word = (254 << 4) - ((((millivolts - 1650) >> 4) * 269) >> 4);
+    code_bytes[0] = code_word >> 8;
+    code_bytes[1] = code_word & 0xff;
+  }
 
   // Send the DAC code word
   if(!dac_start(mask, /*read=*/false))
@@ -86,8 +88,10 @@ bool iobuf_set_voltage(uint8_t mask, __xdata const uint16_t *millivolts_ptr) {
   if(!i2c_stop())
     return false;
 
-  // Enable LDO(s)
-  IOD |= pin_mask;
+  if(millivolts != 0) {
+    // Enable LDO(s)
+    IOD |= pin_mask;
+  }
 
   return true;
 }