ST AN3070 Application note

AN3070

Application note

Managing the Driver Enable signal for RS-485 and IO-Link communications with the STM32™’s USART

Introduction

RS-485 and IO-Link are half-duplex communication protocols that offer easy ways of implementing the physical layer in industrial networks.

The STM32F10x, which comes with up to 5 UART interfaces and features fast DMA transfer and low interrupt latency, meets the RS-485 and IO-Link timing specifications.

This application note aims at providing timing measurements of the DE signal (Driver Enable) switching by using two different methods for managing this signal in RS-485 and IO-Link master transmission.

The application note is organized into three parts:

●it first explains why the timing of the DE signal is critical

●it then describes the two methods used to manage the DE signal

●and, finally, it gives different measurements of the DE signal switching time

January 2010

Doc ID 16312 Rev 1

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Contents	AN3070

Contents

1	DE signal timing constraint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		5
2	Description of the methods used to manage the DE signal . . . . . . . . .		6
	2.1	Method using the DMA interrupt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
	2.2	Method using the USART interrupt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
3	Measuring the DE signal switching time using the
	two methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		8

3.1 Measuring the DE signal switching time . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.1.1Measuring the DE signal switching time using the DMA interrupt

method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

	3.1.2	Measuring the DE signal switching time using the USART interrupt
		method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 9
4	Conclusion .	. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10
5	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		11

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AN3070	List of tables

List of tables

Table 1. Timing measurements of DE switching at 72 MHz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Table 2. Timing measurements of DE switching at 24 MHz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Table 3. Timing measurements of DE switching at 72 MHz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Table 4. Timing measurements of DE switching at 24 MHz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Table 5. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

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List of figures	AN3070

List of figures

Figure 1. DE timing constraint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Figure 2. DMA interrupt method to control the DE signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 3. USART interrupt method to control the DE signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Figure 4. Zoom in DE signal switching period . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

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