tests/pyb/adc.py: Fix test so that it really does test ADC values.

Reading into a bytearray will truncate values to 0xff so the assertions
checking read_timed() would previously always succeed.

Thanks to @peterhinch for finding this problem and providing the solution.

tests/pyb/adc.py

@@ -1,33 +1,34 @@
-from pyb import ADC
-from pyb import Pin
+from pyb import ADC, Timer
 
-pin = Pin('X22', mode=Pin.IN, pull=Pin.PULL_DOWN)
-adc = ADC('X22')
-print(adc)
+adct = ADC(16) # Temperature 930 -> 20C
+print(adct)
+adcv = ADC(17) # Voltage 1500 -> 3.3V
+print(adcv)
 
-# read single sample
-val = adc.read()
-assert val < 500
+# read single sample; 2.5V-5V is pass range
+val = adcv.read()
+assert val > 1000 and val < 2000
 
 # timer for read_timed
-tim = pyb.Timer(5, freq=500)
+tim = Timer(5, freq=500)
 
 # read into bytearray
-buf = bytearray(50)
-adc.read_timed(buf, tim)
+buf = bytearray(b'\xff' * 50)
+adcv.read_timed(buf, tim)
 print(len(buf))
 for i in buf:
-    assert i < 500
+    assert i > 50 and i < 150
 
 # read into arrays with different element sizes
 import array
-ar = array.array('h', 25 * [0])
-adc.read_timed(ar, tim)
-print(len(ar))
-for i in buf:
-    assert i < 500
-ar = array.array('i', 30 * [0])
-adc.read_timed(ar, tim)
-print(len(ar))
-for i in buf:
-    assert i < 500
+arv = array.array('h', 25 * [0x7fff])
+adcv.read_timed(arv, tim)
+print(len(arv))
+for i in arv:
+    assert i > 1000 and i < 2000
+
+arv = array.array('i', 30 * [-1])
+adcv.read_timed(arv, tim)
+print(len(arv))
+for i in arv:
+    assert i > 1000 and i < 2000

tests/pyb/adc.py.exp

@@ -1,4 +1,5 @@
-<ADC on X22 channel=13>
+<ADC on 16 channel=16>
+<ADC on 17 channel=17>
 50
 25
 30