programacion icsp

TB013How to Implement ICSP™ Using PIC16CXXX OTP MCUs

INTRODUCTION

In-Circuit Serial Programming (ICSP™) is a great wayto reduce your inventory overhead and time-to-marketfor your product. By assembling your product with ablank Microchip microcontroller (MCU), you can stockone design. When an order has been placed, theseunits can be programmed with the latest revision offirmware, tested, and shipped in a very short time. Thismethod also reduces scrapped inventory due to oldfirmware revisions. This type of manufacturing systemcan also facilitate quick turnarounds on custom ordersfor your product.

Most people would think to use ICSP with PICmicro™OTP MCUs only on an assembly line where the deviceis programmed once. However, there is a method bywhich an OTP device can be programmed severaltimes depending on the size of the firmware. Thismethod, explained later, provides a way to fieldupgrade your firmware in a way similar to EEPROM- orFlash-based devices.

HOW DOES ICSP WORK?

Now that ICSP appeals to you, what steps do you taketo implement it in your application? There are threemain components of an ICSP system: ApplicationCircuit, Programmer and Programming Environment.

Author: Rodger RicheyMicrochip Technology Inc.

1997 Microchip Technology Inc. Prelimin

Application Circuit

The application circuit must be designed to allow all theprogramming signals to be directly connected to thePICmicro. Figure 1 shows a typical circuit that is astarting point for when designing with ICSP. Theapplication must compensate for the following issues:

1. Isolation of the MCLR/VPP pin from the rest ofthe circuit.

2. Isolation of pins RB6 and RB7 from the rest ofthe circuit.

3. Capacitance on each of the VDD, MCLR/VPP,RB6, and RB7 pins.

4. Minimum and maximum operating voltage forVDD.

5. PICmicro Oscillator.6. Interface to the programmer.

The MCLR/VPP pin is normally connected to an RC cir-cuit. The pull-up resistor is tied to VDD and a capacitoris tied to ground. This circuit can affect the operation ofICSP depending on the size of the capacitor. It is,therefore, recommended that the circuit in Figure 1 beused when an RC is connected to MCLR/VPP. Thediode should be a Schottky-type device. Another issuewith MCLR/VPP is that when the PICmicro device isprogrammed, this pin is driven to approximately 13Vand also to ground. Therefore, the application circuitmust be isolated from this voltage provided by theprogrammer.

FIGURE 1: TYPICAL APPLICATION CIRCUIT

Application PCBPIC16CXXX

MCLR/VPP

VDD

VSS

RB7RB6

VDD VDD

To application circuit

Isolation circuits

ICSP Connector

PRO MATE and PICSTART are registered trademarks and PICmicro and ICSP are trademarks of Microchip Technology Inc.

ary DS91013B-page 1

TB013

Pins RB6 and RB7 are used by the PICmicro for serialprogramming. RB6 is the clock line and RB7 is the dataline. RB6 is driven by the programmer. RB7 is abi-directional pin that is driven by the programmer whenprogramming, and driven by the PICmicro when verify-ing. These pins must be isolated from the rest of theapplication circuit so as not to affect the signals duringprogramming. You must take into consideration theoutput impedance of the programmer when isolatingRB6 and RB7 from the rest of the circuit. This isolationcircuit must account for RB6 being an input on thePICmicro, and for RB7 being bi-directional (can bedriven by both the PICmicro and the programmer). Forinstance, PRO MATE® II has an output impedance of1k¾. If the design permits, these pins should not beused by the application. This is not the case with mostapplications so it is recommended that the designerevaluate whether these signals need to be buffered. Asa designer, you must consider what type of circuitry isconnected to RB6 and RB7 and then make a decisionon how to isolate these pins. Figure 1 does not showany circuitry to isolate RB6 and RB7 on the applicationcircuit because this is very application dependent.

The total capacitance on the programming pins affectsthe rise rates of these signals as they are driven out ofthe programmer. Typical circuits use several hundredmicrofarads of capacitance on VDD which helps todampen noise and ripple. However, this capacitancerequires a fairly strong driver in the programmer tomeet the rise rate timings for VDD. Most programmersare designed to simply program the PICmicro itself anddon’t have strong enough drivers to power the applica-tion circuit. One solution is to use a driver boardbetween the programmer and the application circuit.The driver board requires a separate power supply thatis capable of driving the VPP and VDD pins with thecorrect rise rates and should also provide enough cur-rent to power the application circuit. RB6 and RB7 arenot buffered on this schematic but may require buffer-ing depending upon the application. A sample driverboard schematic is shown in Appendix A.

The Microchip programming specification states thatthe device should be programmed at 5V. Special con-siderations must be made if your application circuitoperates at 3V only. These considerations may includetotally isolating the PICmicro during programming. Theother issue is that the device must be verified at theminimum and maximum voltages at which the applica-tion circuit will be operating. For instance, a battery

Note: The driver board design MUST be tested inthe user’s application to determine the effectsof the application circuit on the program-ming signals timing. Changes may berequired if the application places a signifi-cant load on VDD, VPP, RB6 OR RB7.

DS91013B-page 2 Prelimin

operated system may operate from three 1.5V cells giv-ing an operating voltage range of 2.7V to 4.5V. Theprogrammer must program the device at 5V and mustverify the program memory contents at both 2.7V and4.5V to ensure that proper programming margins havebeen achieved. This ensures the PICmicro option overthe voltage range of the system.

This final issue deals with the oscillator circuit on theapplication board. The voltage on MCLR/VPP must riseto the specified program mode entry voltage before thedevice executes any code. The crystal modes availableon the PICmicro are not affected by this issue becausethe Oscillator Start-up Timer waits for 1024 oscillationsbefore any code is executed. However, RC oscillatorsdo not require any startup time and, therefore, theOscillator Startup Timer is not used. The programmermust drive MCLR/VPP to the program mode entry volt-age before the RC oscillator toggles four times. If theRC oscillator toggles four or more times, the programcounter will be incremented to some value X. Nowwhen the device enters programming mode, the pro-gram counter will not be zero and the programmer willstart programming your code at an offset of X. Thereare several alternatives that can compensate for a slowrise rate on MCLR/VPP. The first method would be tonot populate the R, program the device, and then insertthe R. The other method would be to have the pro-gramming interface drive the OSC1 pin of the PICmicroto ground while programming. This will prevent anyoscillations from occurring during programming.

Now all that is left is how to connect the application cir-cuit to the programmer. This depends a lot on theprogramming environment and will be discussed in thatsection.

Programmer

The second consideration is the programmer.PIC16CXXX MCUs only use serial programming andtherefore all programmers supporting these deviceswill support ICSP. One issue with the programmer isthe drive capability. As discussed before, it must beable to provide the specified rise rates on the ICSP sig-nals and also provide enough current to power theapplication circuit. Appendix A shows an exampledriver board. This driver schematic does not show anybuffer circuitry for RB6 and RB7. It is recommendedthat an evaluation be performed to determine if buffer-ing is required. Another issue with the programmer iswhat VDD levels are used to verify the memory contentsof the PICmicro. For instance, the PRO MATE II verifiesprogram memory at the minimum and maximum VDD

levels for the specified device and is therefore consid-ered a production quality programmer. On the otherhand, the PICSTART® Plus only verifies at 5V and is forprototyping use only. The Microchip programmingspecifications state that the program memory contentsshould be verified at both the minimum and maximumVDD levels that the application circuit will be operating.This implies that the application circuit must be able tohandle the varying VDD voltages.

ary 1997 Microchip Technology Inc.

How to Implement ICSP™ Using PIC16CXXX OTP MCUs

There are also several third party programmers that areavailable. You should select a programmer based onthe features it has and how it fits into your programmingenvironment. The Microchip Development SystemsOrdering Guide (DS30177) provides detailed informa-tion on all our development tools. The Microchip ThirdParty Guide (DS00104) provides information on all ofour third party tool developers. Please consult thesetwo references when selecting a programmer. Manyoptions exist including serial or parallel PC host con-nection, stand-alone operation, and single or gang pro-grammers. Some of the third party developers includeAdvanced Transdata Corporation, BP Microsystems,Data I/O, Emulation Technology and Logical Devices.

Programming Environment

The programming environment will affect the type ofprogrammer used, the programmer cable length, andthe application circuit interface. Some programmersare well suited for a manual assembly line while othersare desirable for an automated assembly line. You maywant to choose a gang programmer to program multiplesystems at a time.

The physical distance between the programmer andthe application circuit affects the load capacitance oneach of the programming signals. This will directlyaffect the drive strength needed to provide the correctsignal rise rates and current. This programming cablemust also be as short as possible and properlyterminated and shielded, or the programming signalsmay be corrupted by ringing or noise.

Finally, the application circuit interface to the program-mer depends on the size constraints of the applicationcircuit itself and the assembly line. A simple headercan be used to interface the application circuit to theprogrammer. This might be more desirable for a man-ual assembly line where a technician plugs theprogrammer cable into the board. A different method isthe use of spring loaded test pins (commonly referredto as pogo pins). The application circuit has pads onthe board for each of the programming signals. Thenthere is a fixture that has pogo pins in the same config-uration as the pads on the board. The applicationcircuit or fixture is moved into position such that thepogo pins come into contact with the board. Thismethod might be more suitable for an automatedassembly line.

After taking into consideration the issues with the appli-cation circuit, the programmer, and the programmingenvironment, anyone can build a high quality, reliablemanufacturing line based on ICSP.


Other Benefits

ICSP provides other benefits, such as calibration andserialization. If program memory permits, it would becheaper and more reliable to store calibration con-stants in program memory instead of using an externalserial EEPROM. For example, your system has a ther-mistor which can vary from one system to another.Storing some calibration information in a table formatallows the micrcontroller to compensate in software forexternal component tolerances. System cost can bereduced without affecting the required performance ofthe system by using software calibration techniques.But how does this relate to ICSP? The PICmicro hasalready been programmed with firmware that performsa calibration cycle. The calibration data is transferredto a calibration fixture. When all calibration data hasbeen transferred, the fixture places the PICmicro inprogramming mode and programs the PICmicro withthe calibration data. Application note AN656, In-CircuitSerial Programming of Calibration Parameters Using aPICmicro Microcontroller, shows exactly how to imple-ment this type of calibration data programming.

The other benefit of ICSP is serialization. Each individ-ual system can be programmed with a unique or ran-dom serial number. One such application of a uniqueserial number would be for security systems. A typicalsystem might use DIP switches to set the serial num-ber. Instead, this number can be burned into programmemory, thus reducing the overall system cost and low-ering the risk of tampering.

Field Programming of PICmicro OTP MCUs

An OTP device is not normally capable of beingreprogrammed, but the PICmicro architecture givesyou this flexibility provided the size of your firmware isat least half that of the desired device and the device isnot code protected. If your target device does not haveenough program memory, Microchip provides a widespectrum of devices from 0.5K to 8K program memorywith the same set of peripheral features that will helpmeet the criteria.

The PIC16CXXX microcontrollers have two vectors,reset and interrupt, at locations 0x0000 and 0x0004.When the PICmicro encounters a reset or interrupt con-dition, the code located at one of these two locations inprogram memory is executed. The first listing ofExample 1 shows the code that is first programmed intothe PICmicro. The second listing of Example 1 showsthe code that is programmed into the PICmicro for thesecond time.

ary DS91013B-page 3


The example shows that to program the PICmicro asecond time the memory location 0x0000, originallygoto Main (0x2808), is reprogrammed to all 0’s whichhappens to be a nop instruction. This location cannotbe reprogrammed to the new opcode (0x2860)because the bits that are 0’s cannot be reprogrammedto 1’s, only bits that are 1’s can be reprogrammed to0’s. The next memory location 0x0001 was originallyblank (all 1’s) and now becomes a goto Main(0x2860). When a reset condition occurs, the PICmicroexecutes the instruction at location 0x0000 which is thenop, a completely benign instruction, and then exe-cutes the goto Main to start the execution of code.The example also shows that all program memory loca-tions after 0x005A are blank in the original program sothat the second time the PICmicro is programmed, therevised code can be programmed at these locations.The same descriptions can be given for the interruptvector at location 0x0004.

This method changes slightly for PICmicros with >2Kwords of program memory. Each of the goto Mainand goto ISR instructions are replaced by the follow-ing code segments due to paging on devices with >2Kwords of program memory.

movlw <page> movlw <page>movwf PCLATH movwf PCLATHgoto Main goto ISR

Now your one time programmable PICmicro is exhibit-ing more EEPROM- or Flash-like qualities.


CONCLUSION

Microchip Technology Inc. is committed to supportingyour ICSP needs by providing you with our many yearsof experience and expertise in developing ICSPsolutions. Anyone can create a reliable ICSP program-ming station by coupling our background with someforethought to the circuit design and programmerselection issues previously mentioned. Your localMicrochip representative is available to answer anyquestions you have about the requirements for ICSP.

ary DS91013B-page 5

TB013

APPENDIX A: SAMPLE DRIVER BOARD SCHEMATIC

R6

1V

PP_O

UT

TO

CIR

CU

IT

3 2 1

41U

1A

TL

E21

44A

R9

100

R9

100

VC

CQ1

2N39

06

R10

100

R2

33k

5 6 7

U1B

TL

E21

44A

VC

C

VC

C

15V

EX

TE

RN

AL

PO

WE

R S

UP

PL

Y R12

100k

VP

P_I

N

FR

OM

PR

OG

RA

MM

ER

C1

1NF

D1

12.7

V

Q2

2N22

22

R13

5k

Q3

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06

C3

0.1µ

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R15

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IRC

UIT

C6

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CR

18

100

R17

100

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5k

R19

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6.2V

VD

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12

13

14U

1D

TL

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10k

R21

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RB

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or R

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*see

text

in te

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rief.

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in te

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cal b

rief.

DS91013B-page 6 Preliminary 1997 Microchip Technology Inc.


NOTES:

ary DS91013B-page 7

Information contained in this publication regarding device applications and the like is intended through suggestion only and may be superseded by updates.It is your responsibility to ensure that your application meets with your specifications. No representation or warranty is given and no liability is assumed byMicrochip Technology Incorporated with respect to the accuracy or use of such information, or infringement of patents or other intellectual property rightsarising from such use or otherwise. Use of Microchip’s products as critical components in life support systems is not authorized except with express writtenapproval by Microchip. No licenses are conveyed, implicitly or otherwise, except as maybe explicitly expressed herein, under any intellectual propertyrights. The Microchip logo and name are registered trademarks of Microchip Technology Inc. in the U.S.A. and other countries. All rights reserved. All othertrademarks mentioned herein are the property of their respective companies.

-page 12 2000 Microchip Technology Inc.

All rights reserved. © 2000 Microchip Technology Incorporated. Printed in the USA. 2/00 Printed on recycled paper.

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