ST7261

Features:

Enable executing 63 basic instructions
Fast 8-bit by 8-bit multiply
17 main addressing modes (with indirect addressing mode)
Two 8-bit index registers
16-bit stack pointer
Low power HALT and WAIT modes
Priority maskable hardware interrupts
Non-maskable software/hardware interrupts



Description

The ST7261 accepts either a Crystal or Ceramic resonator, or an external clock signal to drive the internal oscillator. The internal clock (fCPU) is derived from the external oscillator frequency (fOSC), by dividing by 3 and multiplying by 2. By setting the OSC12/6 bit in the option byte, a 12 MHz external clock can be used giving an internal frequency of 8 MHz while maintaining a 6 MHz clock for USB (refer to Figure 10).

The internal clock signal (fCPU) consists of a square wave with a duty cycle of 50%. ST7261 is further divided by 1, 2, 4 or 8 depending on the Slow Mode Selection bits in the Miscellaneous register (SMS[1:0])

The internal oscillator is designed to operate with an AT-cut parallel resonant quartz or ceramic resonator in the frequency range specified for fosc.

The ST7261 shown in Figure 9 is recommended when using a crystal, and Table 3 lists the recommended capacitors. The crystal and associated components should be mounted as close as possible to the input pins in order to mini-mize output distortion and start-up stabilization time.