The measurement range of the ADC is from 0 to +5 volts. The desired range of the analog signal which is being measured is from -15V to +15V. A simple op amp circuit was developed to reduce the amplitude of the analog signal, and shift its voltage for compatibility with the ADC.
BlackBox One uses an HC11 evaluation board, also from Motorola. This
pc board includes a serial connector, a RS-232 level converter, a power
connector, and a header which provides access to all of the pins on the
HC11 for prototyping. This header was used to connect the scaled analog
signal to the HC11, and the serial port was used for communication with
the host. In this case, the host is a Palm-compatible PDA running
the Palm-O-Scope application.
BlackBox One implements the simplifed BlackBox protocol. In this protocol, the host requests a new set of samples by transmitting a single byte. When the HC11 receives a byte from its serial port, it moves to the next state, waiting for a trace. In this state, the software constantly polls the data from the ADC, waiting to see the signal rst go negative, then cross zero. This ensures detection of a rising edge, triggered at 0 volts.
As soon as the desired edge is detected, BlackBox One begins transmitting
samples from the ADC. BlackBox One's sample rate is entirely determined
by the serial transmission rate. The BlackBox protocol specifes that all
samples are 16 bits, or two bytes, in length. At 9600 bps, or 960 bytes
per second, the sample rate is 480 Hz, which is very slow. After 100 samples
have been transmitted, the software returns to waiting for a request from
the host. This cycle continues forever.
BlackBox One is very slow. It was developed as a demonstration tool, to show the functionality of the Palm-OScope. BlackBox Two, which was actually designed before BlackBox One, is intended to provide a more usable oscilloscope system. BlackBox Two uses an external ADC instead of the ADC built into the HC11. This ADC is capable of producing samples every microsecond, for a 1.0 MHz sample rate. This ADC also produces 8-bit samples, instead of the desired 16-bits.
With the ADC producing samples at 1 MHz, the HC11 is too slow to record
the samples in memory, much less transmit them to the host. Instead,
a fifo buffer was developed in hardware, using an SRAM, a counter and some
logic. The fifo allows the ADC to store 100 samples in the SRAM at
1 MHz, and later allows the HC11 to read the samples from the SRAM at a
much slower speed.