BitScope "Picard" PIC

The original BitScope is based on the MicroChip PIC 16F84 we call Picard.

Its fast RISC instruction set and the tri-state I/O pins make it ideal for BitScope.

Pic Chip.

At first glance you may think that an 18 pin PIC does not have enough pins for a useful design.

However, with careful planning it is possible to use most pins for more than one function which makes good use of the features of this innovative microcontroller while keeping the construction costs down.

The key is to reuse several of the pins depending on the hardware operating mode. In BitScope's case, two modes are defined; COUNT and SHIFT.

Here is a brief description of the pin functions:

PIN Function Description

Range 0
Range 1
COUNT mode:
Output pins that control the Analog gain of the Y amplifiers.
  • 0 Gain = 4.583
  • 1 Gain = 1.000
  • 2 Gain = 0.500
  • 3 Gain = 0.190
Note that the gain of the ADC Buffer is 1.667
SHIFT mode:
I/O pins that are available at the LOGIC POD for external Smart Ports. In COUNT mode, analog switches isolate these signals from the LOGIC POD.

RA2 Channel A/B Output pin that swiches the Analog source between CHA BNC and CHB BNC, or in POD mode between PODA or PODB analog signals brought from the LOGIC POD.

RA3 zz-clk "The clock that sleeps". Tri-state I/O pin. State machines in PLD devices should not be exposed to glitches on the clock. Some registers may see the clock, others may may not, resulting in non-deterministic behavior. Jamming a 50MHz clock signal is not polite. U6A (74AC74) is double clocked by frequency doubler U3D (74AC86) (on the clean edge) and allows the PIC to have 3 state synchronous control via RA3. High, Low, Freerunning - no glitches. This control is important as we must use zz-clk to shift in control words to the PLD as well as modulate the sampling for lower frequency timebase measurements.

RA4 Digital Data Input connects to U5 8:1 MUX to read the 8 data bits of the Logic Analyzer RAM bus. Bit7 is routed through Spock to be optionally the TRIGGER MATCH or FREQUENCY/EVENT signals. RA4 may be connected to the PIC prescaler in the OPTION register

RB0 Analog Data COUNT mode:
Input connects to U4 8:1 MUX to read the 8 data bits of the ADC RAM Bus. As RB0 is the PIC INT source, during SAMPLE, if the MUX points to Bit7 then INT may be used as a zero crossing detector for Analog TRIGGER (in additional to the Complex trigger implemented in the PLD)
Spock Data In SHIFT mode:
Output signal is Shift-In data to feed Spock which needs 5 bytes to set up counters, trigger bytes and options.

RB1 SEL 0 COUNT mode:
Output A0 for data MUXs U4, U5.
Spock Data Out SHIFT mode:
Input Shift-Out data from Spock. Current 16 bit counter state in Spock is shifted out as new data is shifted in.

Output pins A1, A2 for data MUXs U4, U5.

RB4 COUNT/!SHIFT Output which controls the hardware mode. That is, COUNT or SHIFT. A value of ONE causes Spock to shift new data into its registers. A value of zero causes Spock to count and trigger.

RB5 Serial Out Serial data output.
RB6 Serial In Serial data Input.

RB7 STORE/!READ Output control for RAM and ADC/Buffer control. This signal selects either read or write for the RAM, allowing SAMPLE data written to RAM, or later read back.

Clock Select The PIC is clocked by a crystal - which depending on the PIC device may be 4MHz, 10 MHz or 20MHz