User manual

Signal Analysis PICtail™ Daughter Board
 2004 Microchip Technology Inc. DS51476A-page 11
1.5.2 Real-time Mode
Real-time mode refers to the Signal Analysis Application mode that commands the
PIC16F684 to perform one analog-to-digital (A/D) conversion from RC0/AN4 I/O port
pin. The 10-bit conversion data is immediately sent to the PIC16C745 for transfer to the
PC USB port. The Signal Analysis Application then displays the data in a real-time strip
chart format.
1.5.3 Acquisition Mode
Acquisition mode refers to the Signal Analysis Application mode that commands the
PIC16F684 to perform a specified number of A/D conversions at a specified speed
(samples per second) and store the results in the serial EEPROMs.
The acquisition process is controlled by Timer0. The configuration command looks up
precalculated Timer0 prescaler and Timer0 values. This ensures Timer0 will generate
an interrupt on time as specified by the speed configuration parameter. When Timer0
overflows, the interrupt is triggered and initiates an analog-to-digital conversion and
restarts Timer0.
Writing to a serial EEPROM consists of writing to a page of memory (32 bytes for the
25LC640) and initiating a Write cycle. A write cycle can take a maximum of 5 ms (3 ms
typical) to complete. Thus, in order to store A/D conversions at high speeds, two serial
EEPROMs are used. The data conversions are stored in the two serial EEPROMS by
alternating (interleaving) between the two devices.
Figure 1-4 graphically illustrates the Acquisition mode data flow.
FIGURE 1-4: PICA2DLAB ACQUISITION MODE DATA FLOW DIAGRAM
ADC
Queuing
Function
(ISR)
Circular Queue
Dequeuing
Function
(Acquisition)
EERead
Synchronous
Serial
EEPROM
(2x25LC640)
Note: The Signal Analysis PICtail Daughter Board is designed as a development
tool rather than laboratory quality equipment. To reduce product costs, a
number of design choices have been made in favor of lower cost over
increased accuracy. These choices may affect the accuracy and
repeatability of measurements. These potential sources of error include:
•The Analog-to-Digital Converter is referenced to V
DD, which is supplied
by the USB cable. The PICkit 1 Signal Analysis PC Application
assumes its 5.00V, however, the USB specifies only 4.75-5.25V.
•Timing of the samples (in Acquisition mode) is controlled by Timer0 and
the internal oscillator on the PICmicro device. Check the
PIC16C745/765 8-Bit CMOS Microcontroller with the USB data sheet
(DS41124), for actual specifications on oscillator tolerances.