This guide is for anyone who has an SC131 that does not work.

Unless otherwise specified the following tests are performed with a low cost digital multimeter.

Results, below, are shown in a table which has a column for each of the following:

• Test – Description of the test
• My unit – Results on my SC131
• Min. – Minimum acceptable value
• Max. – Maximum acceptable value

Perform each check in the order described and, if possible, resolve any problem found before moving on to the next check.

Visual Inspection

From my experience I think the most likely problem will be an assembly issue. These can often be found with visual inspection, so take your time and carefully do the following:

• Check components are in the correct locations and are fitted the correct way around.
• Look for any IC leg that is bent under the chip and not making contact with the socket.
• With a magnifying glass (or similar) inspect all solder joints looking for any that may have a short to other components or where the solder has not flowed nicely onto the pad and component lead.

Never underestimate the importance of visual inspection.

Jumpers

The following tests assume a jumper shunt is fitted in the position shown below.

The write protection jumper (JP1) should have a shunt fitted in the read only (‘Rd’) position.

Power supply

With the board not connected to a power source and nothing else connected, use a multimeter to check there is not a short on the power lines. The meter reading may take a while to settle due to capacitance.

Test	My unit	Min.	Max.
Power supply resistance measured from U5 pin 16 to U5 pin 8.	> 1 Mꭥ	10 kꭥ	n/a

A value below the minimum would likely indicate a short circuit, such as a solder splash. A less likely cause would be a faulty component, such as a decoupling capacitor.

The following tests are carried out with SC131 powered from an FTDI style USB to serial adapter.

Test	My unit	Min.	Max.
Power supply voltage measured from U5 pin 16 to U5 pin 8.	4.84 V	4.75 V	5.25 V

If you have a means of measuring the power supply current, do so now.

Test	My unit	Min.	Max.
Power supply current running RomWBW	70 mA	30 mA	90 mA

A value below the minimum suggested value might indicate a broken track such that power is not reaching all of the components. A value around the minimum suggested value could be caused by the oscillator not running or the processor being held in reset, so that there is very little activity on the board, and thus low current consumption.

A value above the maximum might be caused by a faulty component, a short circuit somewhere, or perhaps an IC inserted the wrong way around.

Reset

The processor won’t run if the reset signal is low.

Test	My unit	Min.	Max.
Voltage from U5 pin 8 (GND) to U5 pin 15 (/RESET) Reset button not pressed	4.8 V	4.0 V	5.25 V
Voltage from U5 pin 8 (GND) to U5 pin 15 (/RESET) Reset button pressed	0.0 V	0.0 V	0.5 V

Check this voltage drops to less than 0.5 volts when the reset button is pressed. When the reset button is released, the reset signal should go high (> 4.0 V) within 1 second.

If the reset signal stays low, there could be a problem around U7 (the voltage supervisor and reset chip), C8, or the reset button (SW1).

Status LED does not light up

Skip this section if the Status LED (LED1) lights when power is applied, then blinks off for about half a second, then turns on and stays on.

If the LED lights but does not then blink off and on as described above, it is likely that there is a fault around IC U5 or the processor is not running the firmware correctly when it comes out of reset. If you can’t see a problem around U5 then skip the rest of this section.

Press and hold the reset button. If the Status LED lights but did not light at power up, then go back and check the reset signal again. If the LED does no light then continue with these tests.

Test	My unit	Min.	Max.
Power supply voltage at IC U5, pins 8 and 16	4.84 V	4.75 V	5.25 V

If the voltage is missing at IC U5 then check bad solder joints on the power tracks.

Test	My unit	Min.	Max.
Voltage from U5 pin 8 (GND) to the resistor R10 (near Status LED) Test end of resistor furthest away from LEDs	0.09 V	0.0 V	0.5 V

If this voltage is above the maximum, then U5 pin 5 is not pulling the signal low. At reset the addressable latch U5 should be cleared. A cleared output on pin 5 should be low. As we have already checked the reset signal it is possible that IC U5 is faulty.

Test	My unit	Min.	Max.
Voltage from U5 pin 8 (GND) to the resistor R10 (near Status LED) Test end of resistor nearest the LEDs	3.06 V	2.5 V	3.5 V

If the voltage is not within the specified range, it is likely the LED is fitted the wrong way around or is faulty. Also, check solder joints and tracks around R10 and LED1.

Clock

The processor won’t run without a clock signal.

Test	My unit	Min.	Max.
Voltage from U5 pin 8 (GND) to to X1 pin 5 (CLKX)	2.46 V	2.0 V	3.0 V

If the voltage is not within the specified range, it is likely the oscillator is faulty, power is not reaching the oscillator, there is a bad joint around X1 or there is a short on the CLKX line.

Self-test LED sequence

Following a reset, the Status LED should light for a about half a of a second, then blink off for a fraction of a second, then light again. This indicates the self test has passed.

More power tests

Check the 5 volt supply reaches the following places by measuring the voltage at the points indicated.

Test	My unit	Min.	Max.
IC U1 between pins 16 and 32	4.84 V	4.75 V	5.25 V
IC U2 between pins 16 and 32	4.84 V	4.75 V	5.25 V
IC U3 between pins 8 and 16	4.84 V	4.75V	5.25 V
IC U4 between pins 36 and 34 This can be tricky so skip this measurement if you are concerned	4.84 V	4.75 V	5.25 V
IC U5 between pins 8 and 16	4.84 V	4.75 V	5.25 V

Signal tests

The results of the following tests should be treated as a rough guide as they will vary depending on the version of firmware running.

Test	My unit	Min.	Max.
IC U2 between pins 16 and 13 (D0)	2.7 V	1.2 V	3.5 V
IC U2 between pins 16 and 14 (D1)	2.4 V	1.2 V	3.5 V
IC U2 between pins 16 and 15 (D2)	2.4 V	1.2 V	3.5 V
IC U2 between pins 16 and 17 (D3)	2.3 V	1.2 V	3.5 V
IC U2 between pins 16 and 18 (D4)	1.9 V	1.2 V	3.5 V
IC U2 between pins 16 and 19 (D5)	2.3 V	1.2 V	3.5 V
IC U2 between pins 16 and 20 (D6)	2.3 V	1.2 V	3.5 V
IC U2 between pins 16 and 21 (D7)	2.1 V	1.2 V	3.5 V

Test	My unit	Min.	Max.
IC U2 between pins 16 and 12 (A0)	2.5 V	1.2 V	3.5 V
IC U2 between pins 16 and 11 (A1)	2.4 V	1.2 V	3.5 V
IC U2 between pins 16 and 10 (A2)	2.5 V	1.2 V	3.5 V
IC U2 between pins 16 and 9 (A3)	2.6 V	1.2 V	3.5 V
IC U2 between pins 16 and 8 (A4)	3.1 V	1.2 V	3.5 V
IC U2 between pins 16 and 7 (A5)	2.8 V	1.2 V	3.5 V
IC U2 between pins 16 and 6 (A6)	2.6 V	1.2 V	3.5 V
IC U2 between pins 16 and 5 (A7)	1.7 V	1.2 V	3.5 V

Test	My unit	Min.	Max.
IC U2 between pins 16 and 27 (A8)	2.4 V	1.2 V	3.5 V
IC U2 between pins 16 and 26 (A9)	3.6 V	1.2 V	3.5 V
IC U2 between pins 16 and 23 (A10)	3.3 V	1.2 V	3.5 V
IC U2 between pins 16 and 25 (A11)	3.4 V	1.2 V	3.5 V
IC U2 between pins 16 and 4 (A12)	1.5 V	1.2 V	3.5 V
IC U2 between pins 16 and 28 (A13)	1.4 V	1.2 V	3.5 V
IC U2 between pins 16 and 3 (A14)	1.5 V	1.2 V	3.5 V
IC U2 between pins 16 and 31 (A15)	4.8 V	1.2 V	3.5 V

Test	My unit	Min.	Max.
IC U2 between pins 16 and 24 (RD)	2.3 V	1.2 V	3.5 V
IC U2 between pins 16 and 29 (/WR)	4.6 V	4.0 V	5.25 V
IC U2 between pins 16 and 22 (CE)	1.9 V	1.2 V	3.5 V

If any of these signals looks wrong, try measuring the voltage when you press and release the reset button.

If the voltage levels do not appear to change during that first second, it is likely the processor is not running the firmware code in the expected way.