In-Circuit Programming with QuickWriter™
V1.3
This is an application note describing how to design your target for proper in-circuit programming with a
QuickWriter.
THE’IDEALS’
The IDEAL programming environment varies widely, depending on whose perspective you take.
Programming hardware’s Ideal
From the Programming hardware’s perspective, the ideal setup is a chip in a socket, with nothing
else attached to the chip. This gives the programming hardware full control over the process,
eliminating any unknowns.
The next best setup is a chip in a target board with all of its signals isolated from the target during
programming.
Firmware Developer’s Ideal
The firmware developer’s ideal would be to ignore programming issues. He would not want to
give up any pins or make any concessions to accommodate in-circuit programming. In-circuit or
in-socket is a production issue.
Hardware Developer’s Ideal
The hardware developer would like to simply attach a programming connector to the appropriate
signals (or even clip a connector over the chip) and forget about it. He would like to use all of the
signals in the target without worrying about what the programmer might need.
THE REALITY
In-circuit programming is more complicated than in-socket programming. It is possible to use a
programmer that was not designed for in-circuit programming if the hardware designer takes
extreme measures to make the device appear to be in-socket. In other words, completely isolate
the device from the target during programming. Although this is possible, it is rarely done. The
reality is that NONE of the IDEALs are ever realized.
In-circuit programming support must be designed into the programming hardware AND the target
hardware.
IMPLEMENTATION
QuickWriter is designed to make In-Circuit programming as hassle free as possible. It allows the
hardware and firmware developers to come as close to their ideals as possible and still do
reliable in-circuit programming. QuickWriter includes several features to support this.
High Dynamic and Static Drive
QuickWriter includes high dynamic current and static current capabilities on MCLR and VCC.
This allows it to over-come capacitance and loads on these circuits.
Inter-locked VPP and VCC enable/disable
QuickWriter monitors the voltage on the MCLR and VCC pins, allowing it to adapt to different
target loads. Each time QuickWriter enables or disables either of these signals, it monitors its
voltage to ensure it has risen or fallen fully before continuing to the next phase of the
programming algorithm. This interlock is crucial to reliable programming with a wide range of
power, load and capacitance variations in the target.
Target Monitoring
QuickWriter checks for error conditions and complains if it sees indications that the target is not
properly designed to do in-circuit programming safely. For example, it verifies that it can pull
MCLR low before starting. A failure here indicates a wiring problem or a very low value pull-up on
MCLR. QuickWriter verifies that when it turns on VCC, it rises to 5V +- .5 V within a reasonable
amount of time. A failure here would indicate improper wiring, excessive capacitance on VCC or
excessive loading on VCC. Similar checks are made when enabling MCLR. Additionally,
QuickWriter verifies that when MCLR is enabled, it does not drive VCC above its nominal level,
indicating that MCLR is too tightly coupled to VCC or that there is a wiring error.
Passive MCLR/VPP and VCC pull-downs
The MCLR and VCC drivers use active HIGH DRIVE but passive LOW DRIVE. This allows the
programmer to safely evaluate the target’s suitability for in-circuit programming before it applies
ANY power to the board. It also permits monitoring of interaction between the two power rails.
DESIGNING FOR IN-CIRCUIT PROGRAMMING
In addition to the in-circuit support built into QuickWriter, there are still a few basic requirements
on the target to make it in-circuit programmable. The following defines the key issues related to
each signal involved in programming. The numbers in () are the in-circuit programming cable pin
numbers. Don’t let the lengthy discussion put you off; it really is pretty straightforward and most
targets require very little effort to make them in-circuit programmable. This discussion just seams
a little lengthy because we are providing the reasons for the requirements as well as the
requirements themselves. There is a condensed ‘design recommendations’ summary at the end.
MCLR/VPP (1)
HIGH VOLTAGE
This signal is driven to over 13V during programming. Your target must be able to tolerate this high
voltage on this signal. The PIC® MCU is designed to accept this voltage of course. However, anything
else attached to this pin probably will be damaged. You will need to isolate the MCLR/VPP line from
anything else in your target that could be damaged by the programming voltages. Possible devices include
a buffer (used to drive the reset to other devices), a power supply monitor/reset chip or a logic output used
to generate the reset in lieu of the standard RC setup.
Other Drivers
If you use logic to reset the chip in normal operation (rather than depend on the RC), then these drivers
must be isolated unless they can tolerate > 13V AND they do not actively drive (HIGH or LOW) the
MCLR line during programming.
Pull-ups
Any pull-up on MCLR must be 10K or larger. This prevents the programming voltage from forcing the
target power supply above its nominal 5V. It also allows QuickWriter to passively pull-down MCLR to a
reset state. If you have a smaller (stronger) pull-up, QuickWriter’s internal 1K pull-down will not be able
to pull MCLR to a valid low. QuickWriter will notice this and complain. If you have an existing target
with a strong pull-up and changing it to 10K is not an option, you can compensate by strengthening
QuickWriter’s pull-down proportionally. For example, if your pull-up is 4.7K (about 2x as strong), adding
a 1K pull-down between MCLR and ground on QuickWriter’s ICSP cable will double our pull-down
strength.
Capacitors
Although capacitors are generally not used on PICmicro® MCU reset circuits, but they can be and
sometimes are used. QuickWriter can tolerate up to 10uF of capacitance from MCLR to ground. The
voltage interlocks will wait until the capacitor is fully charged/discharged before continuing. NOTE that
this cap must be rated for > 13V!
Devices with Internal MCLR
Some devices require that MCLR be driven BEFORE VCC. This is very unusual and damaging to most
chips. For these devices, the programmer will drive 13V to the MCLR pin before applying power to the
target. If you have anything at all connected to MCLR, you should evaluate its tolerance to 13V
EVEN
WHEN IT IS NOT POWERED.
For these devices, you will want to diode isolate MCLR from VCC so
that MCLR does not attempt to power the VCC rail. Generally, place a diode in series with the MCLR
pull-up (anode to VCC) or completely isolate MCLR from VCC with a jumper.
VCC (2)
Who powers the target?
It is possible to do in-circuit programming with the programmer powering the target or with the target
powering itself. There are trade-offs and limitations with each choice.
Programmer Powered
The programmer can provide up to 100ma of power to power up your target during programming. This
gives it complete control over programming. However, there are a few things to watch for.
Back-feeding Regulators
Most power regulators regulate by adjusting how much current they DRIVE into the circuit. Few are
capable of sinking current. They depend on the load itself to sink current. Since they are not designed to
sink current, many regulators can be damaged if their OUTPUT becomes higher than their INPUT pin.
This is exactly what happens if you allow the programmer to power the target and do not power the input to
the regulator. Most 3-terminal regulator design guides specify if the regulator is tolerant of this condition
or how to design to make it tolerant.
Brown-outs
It is possible that the programmer can provide enough current to power up the target until actual
programming begins. At that time, the additional power required for programming could cause the entire
target board voltage to drop and the programming to fail. Alternatively, the average voltage could stay
valid, but noise could appear on VCC in response to surge current requirements by the circuit of device
being programmed. Either symptom would be an indication that the board requires more than the 100ma of
current the programmer is capable of supplying.
Partially powered targets
If you allow the programmer to back-feed the VCC power rail on the target, that would power up VCC, but
may leave other power rails on the target un-powered. This could be damaging to some components.
For example, if you have a 12 volt power supply powering some devices and this is normally regulated
down to 5V for the PICmico MCU and others, then the 12V devices would not get powered when the
programmer drives current into the VCC (5V) rail. This could cause 5V to be driven into some un-powered
devices.
Similar problems can occur even in 5V only systems if they have multiple isolated power rails.
Capacitance
QuickWriter can drive up to 1000uF on the VCC rail. If there is more capacitance than this,
QuickWriter will report that VCC failed to rise fast enough and will abort programming.
The larger the VCC capacitance, the longer it takes for them to bleed down after programming is
complete, depending on the target load. Large capacitance could increase programming times. If
this becomes a problem, you can add a bleed resister between VCC and GROUND on the ICSP
cable to speed up the bleed-down. Keep the VCC load under 100ma (target + bleed resister.)
Self Powered
This is often the best choice. It eliminates any concern over back-feeding power supplies or about
multi-voltage targets being partially powered. It most closely mimics the target’s intended
operational modes. However, there is one situation in which programmer powered is better.
Internal MCLR
We normally use MCLR to hold the chip in reset until we are ready to enter programming mode.
At that time, we quickly drive MCLR to VPP levels. To properly enter programming mode, we
must drive MCLR to VPP before the oscillator has run more than a few dozen cycles. Otherwise,
the device begins to run and advances the program counter before we get into programming
mode. The result is that we start programming beyond the start of code space, causing a failure.
Some devices have the option of configuring them for internal MCLR, freeing up a pin for general
I/O usage. However, once the pin is programmed as an I/O pin instead of a MCLR function, we
can no longer hold the chip in reset. It simply starts running as soon as it is powered. This
makes it much more difficult to ensure we get MCLR up to VPP levels before the PC starts
advancing, particularly during in-circuit programming.
Microchip solved this ‘catch-22’ by changing the programming specs for these devices so that we
apply VPP BEFORE VCC. This forces the chip into programming mode even if its MCLR function
has been internalized, but ONLY if we can control VCC as well as VPP.
For these devices, it is desirable to allow the programmer to control VCC so that it can properly
enter programming mode, even if the device has been previously programmed to use internal
MCLR. You can further increase the probability of success by enabling the device’s power up
timer. This causes the device to wait longer before starting to execute code.
If other limitations preclude powering the target from the programmer, you could still program
factory fresh devices. You just may not be able to re-program them or to do another verify AFTER
the fuses are programmed. During normal programming, we program and verify everything but
the fuses and THEN program the configuration fuses. The configuration fuses are the last things
we program so we maintain control over MCLR until the very last operation. Once the fuses are
programmed to internalize MCLR, we would loose control over the device.
Another possibility might be to use the programmer’s VCC to control a small relay that actually
powers the target. Make sure the relay coil uses less than 100ma at 5VDC and that you provide
a reverse biased diode across the coil to absorb any inductive ‘kick’ when the relay is opened.
GND (3)
Always connect the programmer’s ground to the target’s ground.
