DSP56857 datasheet(389/662 Pages) FREESCALE

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Introduction

Time-Of-Day (TOD), Rev. 4

Freescale Semiconductor

14-3

14.1 Introduction

The 5685x devices in this manual have a Time-of-Day (TOD) feature implemented as a sequence

of counters to track elapsed time. The hardware is capable of tracking time up to 179.5 years, or

65,535 days.

TOD is comprised of a series of counters tracking elapsed seconds, minutes, hours, and days. The

module requires an input clock ranging from 1-65536Hz integer values only. The clock is further

scaled down to generate a 1Hz clock driving all of the time counters.

All of the time counters are loaded with the time-of-day upon enabling TOD. Time counters

subsequently track the elapsed time. If required, the TOD feature can issue an alarm whenever

selected current time registers match the time programmed into the enabled TOD alarm registers.

The TOD can also provide an interrupt every second.

An appropriate external clock frequency must be provided and the CGM module must be

configured so TOD clock input in integer HZ from 1-65536Hz is provided. Once completed, the

internal clock prescaler within the TOD module can be configured to provide a 1Hz time-base

clock to the module.

The CGM provides a bit to select one of two external clock prescalers. The oscillator TOD clock

prescaler is a fixed

⁄

128 divider. The CGM TOD clock prescaler is a programmable divider

followed by a fixed

⁄

2 divider. The programmable portion ranges from divide by 1 up to 2 to

the 12th power.

The oscillator prescaler is provided for use with a low frequency (2 - 4MHz) external clock and it

is provided for use in conjunction with the PLL. The CGM clock prescaler consumes more power

and it is larger for use with direct external clock frequencies from 4MHz up to 240MHz.

It is preferable to use the highest frequency TOD clock input possible therefore to do as much

clock division as possible using the internal prescaler within the TOD module. TOD clock

frequencies below 10Hz are not recommended.

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