diff --git a/adcext.c b/adcext.c
new file mode 100644
index 0000000..eb80f2a
--- /dev/null
+++ b/adcext.c
@@ -0,0 +1,36 @@
+/* adcext.c -- read external resistor value using ADC */
+/* Copyright (c) 2021-2025 Renaud Fivet */
+
+#include <limits.h>
+#include <stdio.h>
+#include "system.h"	/* uptime, yield(), adc_init(), adc_convert() */
+
+#define RREF 10010  /* Rref is 10kOhm, measured @ 10.01 kOhm */
+
+int main( void) {
+    unsigned last = 0 ;
+    const unsigned short *calp ;        /* TS_CAL, VREFINT_CAL */
+
+/* Initialize ADC and fetch calibration values */
+    calp = adc_init( 2 | (1 << 17)) ;   /* ADC read on GPIOA1 and VREF */
+    short Vcal = calp[ 1] ;             /* VREFINT_CAL */
+
+    printf( "factory calibration: %u, %u, %u\n", calp[ 1], calp[ 0], calp[5]) ;
+
+    for( ;;)
+        if( uptime == last)
+            yield() ;
+        else {
+            short Rsample, Vsample ;
+
+            last = uptime ;
+            Vsample = adc_convert() ;
+            Rsample = adc_convert() ;
+            printf( "%i, %i, %i, ", Vcal, Vsample, Rsample) ;
+            int res = Rsample ? RREF * 4095 / Rsample - RREF : INT_MAX ;
+            Vsample = 3300 * Vcal / Vsample ;
+            printf( "%i, %i.%i\n", res, Vsample / 1000, Vsample % 1000) ;
+        }
+}
+
+/* end of adcext.c */