Download Resolving the error related “Incorrect Voltage at MCRL pin”

Resolving the error related “Incorrect Voltage at MCRL pin”.
During the process of programming the MCRL pin is used to deliver the high-voltage
programming pulse (around 13 Vp-p). The programmer board has a charge-pump
inverter circuit that generates this high voltage from the 5volt supply voltage.
Furthermore, to ensure the correct programming voltage, the programmer employs a feed
back circuit that verifies the programming voltage via its A/D converter circuits. When
one of these circuits fails to operate correctly, it reports the “Incorrect voltage at MCRL
pin” error.
Usually there are three major reasons for this fault.
1. Faulty charge-pump circuit or the high voltage regulator
2. Incorrect sense voltage in the feed-back circuit
3. Incorrect reference voltage for feedback A/D converter
An initial estimate of the fault can be made by looking at the VPP voltage as seen by the
MPLAB software. First connect the circuit to a computer in the usual way. Set the
development mode to MPLAB ICD to open the programming dialog window. Then click
on the “Options” button to open the advanced programming option dialog.
The charge-pump circuit operates only when it is requested by the MPLAB software.
The simplest method to start this circuit is to click on the “Update” button in the
advanced programming options dialog.
Click on the update button and observe the reading that appears for the VPP on the
advanced programming options dialog.
VPP reading
Less than 7V
12.8V – 14.8V
16V – 18V
22V and above
Faulty circuit
Faulty charge-pump circuit.
No fault.
Incorrect reference voltage at A/D converter
Incorrect sense voltage in feedback circuit
Testing for Charge-Pump Circuit.
1. Connect an oscilloscope to TP21 on programmer board. Click on the update
button and observe the voltage. If the charge-pump inverter is operational you
should see a reading around 18V on the oscilloscope. If not check the
components L1, R28, D4 and Q4
2. Next is to check the high voltage regulator. Connect the oscilloscope to TP13 and
then click on the update button. If the regulator is operational you should a read a
value around 13V. Otherwise check U3, Q4 and Q3.
Checking for the Sense voltage in feed back circuit (this can be done only
when the charge pump circuit is operational)
1. Connect the oscilloscope to Pin 3 of U2 and click on the Update button. If the
voltage is correct you should observe a reading around 1.1V. If the reading is
above 1.3V, change R22 with a 10K resistor.
Testing for the Correct Reference voltage for A/D converter.
1. Using an accurate multi-meter measure the voltage at pin 5 of U2. The reading
should be close to 2.06 Volts. However, in some circuits you may read a value
less than 1.90V. This if often due the LED drawing a larger correct and thus the
problem can be corrected by adding an additional resistor in series with the LED.
You may compute the value of this additional resistance as follows:
 2.06  Vref 
  R5
Rnew  
 Vcc  Vref  


where Vref is the voltage at pin5 of U2 and Vcc is the voltage at pin 20 of U2.
The nominal value of R5 is 330Ohms (however it is advisable to remove the
resistor form the circuit and then to measure its exact value using a multi-meter.
Usually the value for new resistor (Rnew) should be between 9 – 16 ohms. You
may round this to the nearest available value.
2. The new resistance should appear in series with the LED. Break the connection
between + terminal of the LED and R36 (on the solder side) of the PCB. Then
solder the new resistor instead of this connection.