Application Note AC261

July 2016 1

Real-Time Clock in Microsemi Fusion FPGAs

Table of Contents

Introduction

Real-time clocks are used in a wide variety of applications. For example, a real-time clock is used to time

stamp particular transactions done on ATM machines or vending machines, such as data transfers.

Real-time clocks is used to record the active run/on time for appliances, industrial machinery, and similar

devices.

Common requirements for real-time clocks include the following capabilities:

• Presetting the real-time clock at any time

• Tracking the time when the application is in standby mode

The Microsemi Fusion

field programmable gate arrays (FPGAs) provide the means to easily implement

a real-time clock within the FPGA with both of the above-mentioned capabilities. The analog block of

Fusion FPGAs contains a real-time counter (RTC) that runs off the analog 3.3 V supply. The RTC block in

Fusion devices contains a 40-bit counter along with a 40-bit match register and a control register that

oversees the functionality of the counter. Figure 1 on page 2 shows an example of implementation of the

match for real-time clock. The clock to the counter is provided by an external crystal. The crystal clock is

pre-scaled (divided by 128) internally before entering the counter. As a result, if the crystal clock is

chosen to 32.768 kHz, 7-bit of the counter is toggle at a rate of one Hz (one second) and the whole

counter equates to just over 136 years of elapsed timekeeping.

The RTC of Fusion FPGAs provides the following capabilities:

• FPGA fabric can read from and write to the 40-bit count register of RTC.

• FPGA fabric can read from and write to the 40-bit match register.

• The overall match output or individual match bits are available to the FPGA fabric.

• FPGA fabric can read from and write to the RTC control register. This enables the user to perform

count enable/disable or counter reset dynamically during normal operation.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Real-Time Clock Preset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2

Real-Time Clock in Standby Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

List of Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Real-Time Clock in Microsemi Fusion FPGAs

2 Revision 2

These characteristics and capabilities of the Fusion RTC make this block an ideal choice for applications

using a real-time clock. By using the RTC block of Fusion devices, the real-time clock application can be

integrated into the FPGA, reducing the cost/size of the application hardware.

Implementation

If the crystal oscillator, driving the RTC block of Fusion FPGAs, is set to 32.768 kHz, 7-bit of the RTC

block is toggle at the rate of one Hz. The contents of the first byte of the RTC count register can be

accessed using the SmartGen IP. 7-bit of the RTC creates a one Hz periodic signal for the FPGA fabric.

This signal can drive specific counters, implemented in the FPGA fabric by users, in which the seconds,

minutes, hours, days, months, or years reside.

Real-Time Clock Preset

The register inside the FPGA fabric holding the real-time clock value can be preset (written) to a user

defined value at any time. Furthermore, the RTC count register can be preset to a user-defined value if

needed, using SmartGen IP. Real-time clock implementations in which only 7-bit of the RTC is used to

create one Hz pulses to the FPGA fabric do not need to write into the RTC. In such applications,

presetting the clock only requires writing into the FPGA fabric registers in which the time is stored.

Real-Time Clock in Standby Mode

In many low power applications, the FPGA fabric enters and exits standby mode by powering down the

1.5 V supply to the FPGA fabric. The RTC block is powered by the external 3.3 V supply, and therefore it

keeps counting during standby mode. Consequently, the content of the RTC provides information of the

elapsed time in standby mode.

If a user application requires the real-time clock to be automatically updated after exiting standby mode,

the current real-time must be stored in the embedded Flash memory before entering standby mode.

When the 1.5 V supply to the FPGA fabric is powered up again, the last content of the real-time clock

registers is retrieved from the embedded Flash memory and adjusted based on the elapsed time

extracted from RTC. Then the updated time can be preset into the FPGA fabric registers and the

real-time clock continues to operate as normal based on the one Hz pulses coming from RTC.

Figure 1 • Generating Match Signal from RTC Register Content