ATA5773C-PXQW - Atmel - Datasheet.Live

ATA5771/73/74: Consideration of External

Circuitry for the RESET pin

1. Introduction

The Atmel® IC ATA5771/73/74 is a UHF transmitter for automotive applications in the

ISM frequency range using Atmel ATtiny44 in one package. Figure 1-1 shows the pin

diagram for the IC and the general function of each pin is described in Table 1-1 on

page 2. This brief application note explains the external circuitry if the RESET function

of the alternate pin PB3 is used in the application.

Figure 1-1. Pinning QFN24 5mm × 5mm

PB3/RESET

PB2

PA7

PB1

PB0

VCC

PA3/T0

PA4/USCK

PA5/MISO

PA2

PA1

PA0

VS_RF

XTAL

GND

GND_RF

ENABLE

GND

ANT2

ANT1

GND

PA_ENABLE

CLK

PA6/MOSI

2322 21 20 19

789101112

Atmel

ATA5771/73/74

Application Note

9214A–RKE–12/10

Atmel ATA5771/73/74

Table 1-1. Pin Description

Pin Symbol Function

1 VCC Microcontroller supply voltage

2 PB0 Port B is a 4-bit bi-directional I/O port with internal pull-up resistor

3 PB1 Port B is a 4-bit bi-directional I/O port with internal pull-up resistor

4 PB3/RESET Port B is a 4-bit bi-directional I/O port with internal pull-up resistor/reset input

5 PB2 Port B is a 4-bit bi-directional I/O port with internal pull-up resistor

6 PA7 Port A is a 4-bit bi-directional I/O port with internal pull-up resistor

7 PA6/MOSI Port A is a 4-bit bi-directional I/O port with internal pull-up resistor

8 CLK Clock output signal for microcontroller. The clock output frequency is set by the crystal to fXTAL/4

9 PA_ENABLE Switches on power amplifier. Used for ASK modulation

10 ANT2 Emitter of antenna output stage

11 ANT1 Open collector antenna output

12 GND Ground

13 PA5/MISO Port A is a 4-bit bi-directional I/O port with internal pull-up resistor

14 PA4/SCK Port A is a 4-bit bi-directional I/O port with internal pull-up resistor

15 PA3/T0 Port A is a 4-bit bi-directional I/O port with internal pull-up resistor

16 PA2 Port A is a 4-bit bi-directional I/O port with internal pull-up resistor

17 PA1 Port A is a 4-bit bi-directional I/O port with internal pull-up resistor

18 PA0 Port A is a 4-bit bi-directional I/O port with internal pull-up resistor

19 GND Microcontroller ground

20 XTAL Connection for crystal

21 VS_RF Transmitter supply voltage

22 GND_RF Transmitter ground

23 ENABLE Enable input

24 GND Ground

GND Ground/backplane on exposed die pad

9214A–RKE–12/10

Atmel ATA5771/73/74

2. External Connection of PB3 As a RESET Pin

The RESET pin (PB3) is active LOW. If the pin is externally set to ground (LOW), a reset of the

microcontroller part is carried out.

The RESET functionality has four purposes:

1. Setting all microcontroller pins to tri-state (except for the XTAL pins)

2. Initializing all I/O registers

3. Setting the program counter to zero

4. Entering programming mode

Furthermore it is possible to enter high-voltage/parallel programming mode by driving the

RESET pin using a “very” high signal voltage between 11.5V and 12.5V. For more information

about this function, please refer to the datasheet.

The reset line normally has an internal pull-up resistor to avoid a floating port. Nevertheless, a

reset can still occur from time to time in noisy environments. A simple approach can be used

to avoid unintentional reset of the microcontroller together with the built-in brown-out circuit.

Figure 2-1 shows the external circuitry on the RESET pin for this purpose.

Figure 2-1.

The external pull-up resistor on the reset line must be selected so that high-voltage program-

ming can be entered while also allowing an ordinary low-level reset to be executed. The

resistor ensures that the reset line does not accidentally revert to low level. If the Atmel®

ATtiny44 microcontroller has to be programmed via an ISP interface using the Atmel STK500,

the pull-up resistor value should not be set too high that the RESET line can no longer be set

to low by the programmer. When using the Atmel STK500 as a programmer, Atmel recom-

mends using a 4.7kΩ resistor as the lowest value. To guarantee the proper functionality of the

debugWIRE, the pull-up resistor value must not go below 10kΩ. The value of the pull-up resis-

tor must therefore be between 10kΩ and 50kΩ.

Reset

GND

VCC

9214A–RKE–12/10

Atmel ATA5771/73/74

A low-pass filter is already internally implemented on the RESET pin to eliminate the spike and

noise which may cause a reset. An additional capacitor can be applied at the RESET pin as a

bypass cap to ground to increase sustainability against noise. The capacitor value of 0.01µF

can be used. Please note that if debugWIRE is used the capacitor value must be lower than

330pF. If high-voltage programming is not used, Atmel recommends adding an ESD protect-

ing diode from the RESET line to VCC. A zener diode can additionally be used to limit the

RESET voltage relative to GND. This is recommended for noisy environments. The external

components mentioned in this section should of course be positioned as closely as possible to

the RESET pin.

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