Datasheet

DS87C520/DS83C520 EPROM/ROM High-Speed Microcontrollers

17 of 43

Crystal/Ring Operation

The DS87C520/DS83C520 allow software to choose the clock source as an independent selection from

the instruction cycle rate. The user can select crystal-based or ring oscillator-based operation under

software control. Power-on reset default is the crystal (or external clock) source. The ring may save

power depending on the actual crystal speed. To save still more power, software can then disable the

crystal amplifier. This process requires two steps. Reversing the process also requires two steps.

The XT/

bit (EXIF.3) selects the crystal or ring as the clock source. Setting XT/

= 1 selects the

crystal. Setting XT/ RG = 0 selects the ring. The RGMD (EXIF.2) bit serves as a status bit by indicating

the active clock source. RGMD = 0 indicates the CPU is running from the crystal. RGMD = 1 indicates it

is running from the ring. When operating from the ring, disable the crystal amplifier by setting the

XTOFF bit (PMR.3) to 1. This can only be done when XT/ RG = 0.

When changing the clock source, the selection will take effect after a one-instruction cycle delay. This

applies to changes from crystal to ring and vise versa. However, this assumes that the crystal amplifier is

running. In most cases, when the ring is active, software previously disabled the crystal to save power. If

ring operation is being used and the system must switch to crystal operation, the crystal must first be

enabled. Set the XTOFF bit to 0. At this time, the crystal oscillation will begin. The

DS87C520/DS83C520 then provide a warm-up delay to make certain that the frequency is stable.

Hardware will set the XTUP bit (STATUS.4) to a 1 when the crystal is ready for use. Then software

should write XT/

to 1 to begin operating from the crystal. Hardware prevents writing XT/

to 1

before XTUP=1. The delay between XTOFF = 0 and XTUP = 1 will be 65,536 crystal clocks in addition

to the crystal cycle startup time.

Switchback has no effect on the clock source. If software selects a reduced clock divider and enables the

ring, a Switchback will only restore the divider speed. The ring will remain as the time base until altered

by software. If there is serial activity, Switchback usually occurs with enough time to create proper baud

rates. This is not true if the crystal is off and the CPU is running from the ring. If sending a serial

character that wakes the system from crystal-less PMM, then it should be a dummy character of no

importance with a subsequent delay for crystal startup.

Figure 3 illustrates a typical decision

set associated with PMM. Table 6 is a summary of the bits relating

to PMM and its operation.