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Flash Microcontroller. S Gupta. Schematic diagram is provided and some of the features of flash microcontroller are highlighted with an emphasis on flash memory reprogramming. Some features of these devices are highlighted below. The entire program memory is made up of flash array. Every single byte of flash program memory can be erased and reprogrammed. And all this can be done by a small user code resident in program memory, without requiring any external higher programming voltage.

This level of flexibility is very beneficial in todays fast pace development, where product updates and modifications are routinely carried out in the field.

Reading and writing to program memory through user code is done through a set of special function registers SFRs. The program memory is not limited to storing code bytes, but can also be used to store 14 bit of coefficient constants. An accidental access to an invalid code byte will result in execution of NOP code, without causing any harm. But unfortunately, it cannot be relied upon and care should be taken to ensure that any coefficient bytes are not accessed as program code bytes.

This is normally suitable for storing coefficient data, which may be programmed from time to time. The Data memory is partitioned into 4 banks of Bytes each. The lower section 00 to 20H of each of these banks is devoted to special function registers, and the balance is left for Data RAM.

This device has an interesting feature that the top 16 Bytes of each bank are used to address the same data memory space 70 to 7FH. This reduces the need for bank switching a real nuisance. This is reasonably good, and 16F87X microcontrollers can vie for a place in industrial control. The conversion rate can be adjusted internally, but for 10 bit results, a conversion time including sample and hold of 20 usec is optimum.

It is also possible to use an external precision reference voltage, but is generally an overkill for this resolution. Accurate counting and timing circuits are becoming a necessity for modern industrial control.

There are three timers on-board, which can also be configured to operate as counters. Timer2 is supported by a period register as well as prescaler and a postscaler. This makes the timer operation very flexible. PWM resolutions of up to bits can be achieved. Capture resolution is claimed to be Yet, a software-UART does not offer the type of response needed for real time control. It is especially true, if the microcontroller is connected in any sort of communication network, where data is constantly flying through the interface.

It has its own dedicated baud rate generator, allowing flexible baud rate to be generated. The choice of a suitable crystal is necessary to achieve reliable operation at high baud rates such as K. These interfaces can also be used for a small scale local area network between a PIC working as a master controller and several other units working as slaves, in close physical proximity. Besides these functions PIC16F87X microcontrollers contains several other peripheral functions, such as watchdog timer, power on reset, brown out reset and sleep mode.

Flash program memory can be reprogrammed either through in-circuit serial programming ICSP interface, or through a user program running in the device itself. It is generally a good idea to provide some sort of protection for accidental erasing of critical sections of code.

PIC16F87X microcontroller also address this issue. A PIC16F87X microcontroller can only be reprogrammed, if certain bits in the configuration register are in correct state. These bits are:. CP1 and CP0 provide code protection for certain blocks of program memory. The following table lists the effect of these two bits on code memory.

Protected Code Memory Words. WRT bit further controls the reprogramming of program memory through user code. All the three bits, even though are resident in code memory space H , are not accessible to internal user program. The bits in configuration register can only be reprogrammed, after entire device is erased.

Thus, the very first time, when the device is programmed the configuration bits have to be set appropriately, so that in circuit programming through either a user code or through ICSP interface can be carried out. There are six special functions registers, which are used in reading and writing to flash program memory.

These are:. It contains the low byte address of program memory location to be accessed. It contains the high byte address of program memory location to be accessed. Since, there is a maximum of 8K words of program memory PIC16F , only a bit address is required. It contains the lower 8 bits of a total of 14 bits program word. It contains the upper 6 bits of program word. These bits are,. This bit dictates, whether program memory or data memory is being accessed. This is a flag status bit and is used to indicate a error in programming.

Programming of every single location is a timed event and any premature termination of this event may result in incorrect or atleast unreliable programmed data. This bit enables the programming of program or data memory.

The bit is set to program write to every single location 14 bit word for program memory and 8 bit of data memory. This bit can only be set but not reset by user code. This is intentionally done to prevent a user code from prematurely terminating the write operation. Acts as a status bit.

This bit needs to be set to carry out a read operation on program or data memory. Even though, the intent may be to write to program a memory location, memory reading is required to verify that programming has been carried out successfully.

The data is normally read and is reprogrammed, if not suitable. Once a code location has been programmed, it is read to verify that programming has been successful. Every single program memory location 14 bit word is read as two bytes of data. The read operation is as follows:. PIC16F87X allows a single program memory location word to be programmed, without any need to erase it. Every program memory location 14 bit word is written as two bytes of data- 14 bits to be precise.

It is recommended that interrupts be disabled during the actual programming phase. Programming of every location through user code requires a very specific sequence of instructions to be carried out. Thus, it is inconceivable that a code word may be inadvertently programmed. The sequence to program a program memory location is as follows:. Click here for Flash Module Schematic Diagram. For very simple applications, the connector can be left out and wires directly soldered to the pc board.

But, in general a connector offers more flexibility. User has a choice of two types of connectors: a right angle pin connector, and a straight pin connector. Right angle connector is useful for vertical installation in a target application. Straight pin connector is convenient for horizontal mounting of the module either on top or bottom of another interface board.

It is also useful in situations, where the module is mounted on a flat panel surface and is connected to interface circuit through ribbon cable. A vast majority of microcontroller applications require a serial communication of some sort. Only three interface lines RX, Tx, and Gnd are used. In this case, IC U2 need not be installed on the board. A separate 3-pin header provides is provided in addition to the main interface connector. It duplicates the three interface signals Rx, Tx, and Gnd.

Microcontroller require a regulated 5 volt dc supply for proper operation. A 5 volt regulator U2 is provided on —board. It requires an unregulated dc supply in the range of 8 — 12 volt.

The unregulated dc is applied through the last two pins of main interface connector. The regulated 5 volt output is also available at the interface connector for driving low-power external circuit.

Diode D1 protects against accidental reversal of power supply. If a regulated 5 v supply is available then, the 5 volt regulator circuit can be left out. A popular clock frequency is 4-MHz, which provides a time base of an even 1-microsecond; convenient in timing applications.

Module has the provision to use either a ceramic resonator, or a crystal with two capacitors. Resonator offers lower total cost but at the expense of reduction in timing accuracy. For precision timing, such required at high baud rates, it is recommended that a crystal be used. In the next issue a flash module will be discussed that can be programmed, erased, and reprogrammed through serial port and tested in user application.

Say goodbye to PIC programmers. For more information on Flash Module: email us. Reprogramming Flash Program Memory Flash program memory can be reprogrammed either through in-circuit serial programming ICSP interface, or through a user program running in the device itself.


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