On-Chip Rc Oscillator; Architectural Overview - Atmel AT90S1200 Datasheet

Figure 2. Oscillator Connections
Note: When using the MCU Oscillator as a clock for an external device, an HC buffer should be connected as indicated in the figure.
Figure 3. External Clock Drive Configuration

On-chip RC Oscillator

An on-chip RC oscillator running at a fixed frequency of 1 MHz can be selected as the MCU clock source. If enabled, the
AT90S1200 can operate with no external components. A control bit - RCEN in the Flash Memory selects the on-chip RC
oscillator as the clock source when programmed ("0"). The AT90S1200 is normally shipped with this bit unprogrammed
("1"). Parts with this bit programmed can be ordered as AT90S1200A. The RCEN-bit can be changed by parallel program-
ming only. When using the on-chip RC oscillator for serial program downloading, the RCEN bit must be programmed in
parallel programming mode first.

Architectural Overview

The fast-access register file concept contains 32 x 8-bit general purpose working registers with a single clock cycle access
time. This means that during one single clock cycle, one ALU (Arithmetic Logic Unit) operation is executed. Two operands
are output from the register file, the operation is executed, and the result is stored back in the register file - in one clock
cycle.
AT90S1200
4
MAX 1 HC BUFFER
HC
C2
XTAL2
C1
XTAL1
GND