Spectrum 128 Keypad

The keypad circuitry is based around a PIC1652 microcontroller that was manufactured by General Instruments. The PIC1652 is an 8-bit microcontroller with RISC-like features and has a 33 single-word instruction set. It supports direct, indirect and relative addressing modes and features 7 special function hardware registers, 2 level deep stack, 12
Spectrum 128 Keypad - schematic

I/O ports with individual direction control, an 8-bit real-time clock/counter with 8-bit programmable prescalar, power-on reset, power saving sleep mode and security fuse for code protection. It has 384x12 words of ROM and 25x8 bytes of RAM. It has a maximum operating frequency of 4MHz. The PIC microcontroller continually monitors the keypad matrix via lines RB3-RB7 and RA0-RA3. Lines RB3-RB7 select the row to read and the keys on this row will be read in via RA0-RA3. The keypad communicates to the Spectrum128 via RB0 and RB1. Line RB1 is the input from the Spectrum128 and the combination of resistor R3 and zener diode D1 converts the input voltages of +12V and -12V to +4. 7V and -0. 7V so that the PIC microcontroller can safely read in the line level. The output line to the Spectrum128 (RB0) is either at 0V or +5V and this is sufficient for the Spectrum 128 to read the line levels in as logic 1 and logic 0 respectively. Although +12V is supplied by the Spectrum128 to power the keypad, this is cut down to 5. 1V via zener diode D2. Resistor R4 and capacitor C1 cause a low pulse to line /MCLR at power up thus resetting the PIC microcontroller. Resistor R1 separates the +12V power line from the 5. 1V dropped across zener diode D2. Resistor R3 performs a similar function separating the +12V or -12V from the 4. 7V or -0. 7V dropped across zener diode D1. Resistor R2 limits the current drawn from the output line in case of a...

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