Datasheet

Thursday, 17 July 2003 MiniProject: Design Aspects Colin K McCord
EEE512J2 – Electronic Product Design Page 27 Chapter 5: System Architecture
words of FLASH program memory, 256 data memory bytes, and 368 bytes of user RAM, PIC16F877 also
features an integrated 8-channel 10-bit Analogue-to-Digital converter. Peripherals include two 8-bit timers,
one 16-bit timer, a Watchdog timer, Brown-Out-Reset (BOR), In-Circuit-Serial Programming™, RS-485 type
UART for multi-drop data acquisition applications, and I2C™ or SPI™ communications capability for
peripheral expansion. Precision timing interfaces are accommodated through two CCP modules and two
PWM modules.” [W15]
The ECG signal (form electrodes place on the chest, arms and legs) is inputted into the ECG amplifier. The
propose of the amplifier is to amplify the signal to a level that the PICs Analogue-to-Digital Converter (ADC)
can sample (e.g. a common mode gain of 1000, hence the 1mV ECG signal is amplified to 1V).
The design specification specifies that there should be at least 2 seconds of data on the CRT display at any
one time. Since time-compressed memory is used to obtain a flicker free trace, and it is known that the size
of the memory is 256 bytes, the sample rate must be 128 Hz, e.g. the RAM will take 2 seconds to fill.
Notice that the block diagram shows an external RAM chip, this chip is optional because the chosen PIC has
more than enough internal RAM. The external RAM chip is for flexibility, as the time-compressed memory
uses 75% of the PICs internal RAM and if complex software processing (automatic diagnostic) of the signal
is required (sometime in the future after the product is released, e.g. software upgrade) there may not be
enough free internal RAM.
A dual digital-to-analogue converter (DAC) is used to convert digital data into analogue voltage. This chip is
controlled by the PIC to manipulate the CRT trace, e.g. X (DAC A) and Y (DAC B) specifies the position of
the spot, this spot moves faster than 50Hz across the screen hence it appears as a solid waveform to the
human eye.
The MAX232 line buzzer converts PIC TTL logic (0V for logic 0, 5V for logic 1) into RS232 logic (12V for
logic 0, -12V for logic 1).
5.2. Partitioning Decisions
Figure 5.2a splits software development into 3 main areas; simultaneous development of all three areas is
possible. Figure 5.2b splits hardware development into 4 main areas: simultaneous development is possible
for 3 of the 4 areas, as PCB design cannot start until the digital and analogue circuit has been designed.
Communication
Protocol
Windows
Application
PIC
Program
Figure 5.2a. Software Partitioning
Digital
Circuit
Analogue
Circuit
Packaging
PCB
Design
Figure 5.2b. Hardware Partitioning
Each partition could be given to a different engineer, with many partitions being developed simultaneously,
allow there will be some overlap between partitions (e.g. 7-segment displays mounted on the PCB board
must line up with holes in plastic case). Each partition can be split into sub-partitions, for example: -