In-system programming: Difference between revisions

Revision as of 08:01, 3 September 2020

Revision as of 11:30, 4 September 2020

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[[Image:Isp headers.svg|thumb|6- and 10-pin AVR ISP headers]]

'''In-system programming''' (ISP), also called '''in-circuit serial programming''' (ICSP), is the ability of some [[programmable logic device]]s, [[microcontroller]]s, and other [[embedded device]]s to be programmed while installed in a complete system, rather than requiring the chip to be programmed prior to installing it into the system. It allows firmware updates to be delivered to the on-chip memory of microcontrollers and related processors without requiring specialist programming circuitry on the circuit board, and simplifies design work. <ref>[https://www.oreilly.com/library/view/designing-embedded-hardware/0596007558/ch01.html''Oreilly, Designing Embedded Hardware, 2nd Edition by John Catsoulis'']</ref>

There are several mutually-incompatible in-system programming protocols for programming [[microcontroller]] devices, including [[PIC microcontroller]]s, [[Atmel AVR|AVRs]], and the [[Parallax Propeller]]. ICSP has been primarily implemented by [[Microchip Technology]] for programming PIC and dsPIC devices.

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== History ==

Starting from the early ‘90s we witnessed an important technological evolution in the architecture of the microcontrollers. At first, they wherewere realized in two possible solutions: with [[Programmable read-only memory|OTP (One Time Programmable)]] or with [[EPROM|EPROM memories]]. In these technologies, memory -erasing process requires the chip to be exposed to as ultraviolet light through a specific window above the package. In 1993 [[Microchip Technology]] introduced the first microcontroller with [[EEPROM|EEPROM memory]]: the PIC16C84. EEPROM memories can be electrically erased. This feature allowed to lower the realization costs by removing the erasing window above the package and initiate In-System Programming technology. With ISP flashing process can be performed directly on the board at the end of the production process. This evolution gave the possibility to unify the programming and functional test phase and in production environments and to start the preliminary production of the boards even if the firmware development has not yet been completed. This way it was possible to correct bugs or to make changes at a later time. In the same year, [[Atmel]] developed the first microcontroller with Flash memory, easier and faster to program and with much longer life cycle compared to EEPROM memories.

Microcontrollers that support ISP are usually provided with pins used by the serial communication peripheral to interface with the programmer, a Flash/EEPROM memory and the circuitry used to supply the voltage necessary to program the microcontroller. The communication peripheral is in turn connected to a programming peripheral which provides commands to operate on the Flash or EEPROM memory.

When designing electronic boards for ISP programming it’s necessary to take into account some guidelines to have a programming phase as reliable as possible. Some microcontrollers with a low number of pins share the programming lines with the I/O lines. This could be a problem if the necessary precautions are not taken into account in the design of the board; the device can suffer the damage of the I/O components during the programming. Moreover, it’s important to connect the ISP lines to [[high impedance]] circuitry both to avoid a damage of the components by the programmer and because the microcontroller often can’tcannot supply enough current to pilot the line. Many microcontrollers need a dedicated RESET line to enter in the Programming Mode. It is necessary to pay attention to current supplied for line driving and to check for presence of [[Watchdog timer|watchdogs]] connected to the RESET line that can generate an unwanted reset and, so, to lead a programming failure. Moreover, some microcontrollers need a higher voltage to enter in Programming Mode and, hence, it’s necessary to check that this value it’s not attenuated and that this voltage is not forwarded to others components on the board.

== Industrial application ==

In-System Programming process takes place during the final stage of production of the product and it can be performed in two different ways based on the production volumes.

In the first method, a connector it’sis manually connected to the programmer. This solution expects the human participation to the programming process that has to connect the programmer to the electronic board with the cable. Hence, this solution is meant for low production volumes.

The second method uses [[Test point|test points]] on the board. These are specific areas placed on the printed board, or [[Printed circuit board|PCB]], that are electrically connected to some of the electronic components on the board. Test points are used to perform functional tests for components mounted on board and, since they are connected directly to some microcontroller poinspins, they are very effective for ISP. For medium and high production volumes using test points is the best solution since it allows to integrate the programming phase in an assembly line.

In production lines, boards are placed on a bed of nails called [[Test fixture|fixture]]. The latter are integrated, based on the production volumes, in semiautomatic or automatic test systems called [[Automatic test equipment|ATE – Automatic Test Equipment]]. Fixtures are specificlyspecifically designed for each board - or at most for few models similar to the board they were designed for – therefore these are interchangeable in the system environment where they are integrated. The test system, once the board and the fixture are placed in position, has a mechanism to put in contact the needles of the fixture with the Test Points on the board to test. The system it’s connected to, or has directly integrated inside, an ISP programmer. This one has to program the device or devices mounted on the board: for example, a microcontroller and/or a serial memory.

== Microchip ICSP ==

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# '''Parallel port''' - large bulky cable, most computers have only one port and it may be inconvenient to swap the programming cable with an attached printer. Most laptops newer than 2010 do not support this port. Parallel port programming is very fast.

# '''Serial port''' (COM port) - At one time the most popular method. Serial ports usually lack adequate circuit programming supply voltage. Most computers and laptops newer than 2010 lack support for this port.

# '''Socket''' (in or out of circuit) - the CPU must be either removed from circuit board, or a clamp must be attached to the chip -making access an issue.

# '''USB cable''' - Small and light weight, has support for voltage source and most computers have extra ports available. The distance between the circuit to be programmed and the computer is limited by the length of USB cable - it must usually be less than 180&nbsp;cm. This can make programming devices deep in machinery or cabinets a problem.

ICSP programmers have many advantages, with size, computer port availability, and power source being major features. Due to variations in the interconnect scheme and the target circuit surrounding a micro-controller, there is no programmer that works with ''''' all ''''' possible target circuits or interconnects. [http://www.Microchip.com Microchip] provides a detailed ICSP programming guide<ref>[http://ww1.microchip.com/downloads/en/devicedoc/30277d.pdf ww1.microchip.com/downloads/en/devicedoc/30277d.pdf ICSP programming guide], Microchip</ref> Many sites provide programming and circuit examples.

PICs are programmed using five signals (a sixth pin 'aux' is provided but not used). The data is transferred using a two -wire synchronous serial scheme, three more wires provide programming and chip power. The clock signal is always controlled by the programmer.

[[File:Icsp communication.jpg|thumb|Typical programming communication]]

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[[File:Icsp-pinouts.png|thumb|Typical chip connections]]

* ''' V_pp ''' - Programming mode voltage. This must be connected to the MCLR pin, or the V_pp pin of the optional ICSP port available on some large-pincountpin-count PICs. To put the PIC into programming mode, this line must be in a specified range that varies from PIC to PIC. For 5{{Nbsp}}V PICs, this is always some amount above V_dd, and can be as high as 13.5 V. The 3.3 V only PICs like the 18FJ, 24H, and 33F series use a special signature to enter programming mode and V_pp is a digital signal that is either at ground or Vdd. There is no one V_pp voltage that is within the valid V_pp range of all PICs. In fact, the minimum required V_pp level for some PICs can damage other PICs.

* ''' V_dd ''' - This is the positive power input to the PIC. Some programmers require this to be provided by the circuit (circuit must be at least partially powered up), some programmers expect to drive this line themselves and require the circuit to be off, while others can be configured either way (like the Microchip ICD2). The Embed Inc programmers expect to drive the V_dd line themselves and require the target circuit to be off during programming.

* ''' V_ss ''' - Negative power input to the PIC and the zero volts reference for the remaining signals. Voltages of the other signals are implicitly with respect to V_ss.