GENERAL DESCRIPTION
The W921E840A/W921C840 is single-chip CMOS 4-bit microcontroller that is subset of W921E880A/W921C880. They features four multi-function timers, one channel DTMF generator, an 8-bit D/A converter circuit, ten interrupt sources, 48-level subroutine nesting, and built in four by one channel comparator circuit. Two power down modes, hold and stop mode, reduce power dissipation. The excellent memory structure, 8K super EPROM in W921E840A and 8K mask ROM in W921C840 for program code and 512 x 4 bit RAM minimize the need for external memory devices.

FEATURES
Operating Voltage
· 2.8 to 5.5V operating voltage for W921E840A EPROM Type
· 2.4 to 5.5V operating voltage for W921C840 Mask ROM Type

Operating Frequency
· Crystal or RC for main system clock
   - Crystal for 400 K, 800 K, 2 M, 3.58 M, 4 MHz
   - RC up to 4 MHz

Memory
· 8K ´ 10-bit ROM (super EPROM)
· 512 ´ 4-bit RAM
   - 64 ´ 4-bit special registers
   - 16 ´ 4-bit working registers
   - 128 ´ 4-bit general registers
   - 304 ´ 4-bit multi-purpose registers

Stack
· 8-bit stack pointer

I/O Pins
· 20 bidirectional and individually controllable I/O lines
   - P2 Port: P2.0 to P2.3 large sink current pins and open drain option
   - P3 Port: P3.0 to P3.3 multi-function I/O
   - P4 Port: P4.0 to P4.3 open drain and pull high resistor option, multi-function I/O
   - P5 Port: P5.0 to P5.3 multi-function I/O
   - P6 Port: P6.0 to P6.3 open drain and pull high resistor option, multi-function I/O
· 14 bidirectional I/O lines
   - PA Port: PA.0 to PA.3 open drain and pull high resistor option
   - PB Port: PB.0 to PB.3 open drain and pull high resistor option
   - PC Port: PC.1 to PC.3 open drain and pull high resistor option
   - PD Port: PD.0 to PD.1 open drain and pull high resistor option

Serial I/O Interface
· Clock synchronous multi-nibbles serial transmitter/receiver interface

DTMF Generator
· One channel DTMF generator

Beep Tone Generator
· One channel beep tone generator

8-bit D/A Converter
· One channel 8-bit D/A converter

Voltage Comparator
· Multiplexed four channel voltage comparator

Timer/Counter
· Timer 0: 2 to 19 order divider
   Auto-reload timer
   Watch-dog timer
· Timer 1: 2 to 19 order divider
   Auto-reload timer
   Arbitrary waveform generator
   External event counter
· Timer 2: 2 to 19 order divider
   Auto-reload timer
   Arbitrary waveform generator
   Period/pulse width measurement function
· Timer 3: 2 to 19 order divider
   Auto-reload timer

Interrupt
· Four external sources: INT0 (P4.3), P4 Port (P4.0 to P4.2)
· Six internal sources: Timer 0, Timer 1, Timer 2, Timer 3, Comparator, Serial Port

Operating Mode (System Clock)
· Normal mode: system clock operating
· HOLD mode: no operation except for oscillator (system clock stops only)
· STOP mode: no operation including oscillator