ATA5771/73/74: Consideration of External Circuitry for the RESET pin 1. Introduction The Atmel(R) 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. Application Note GND ENABLE GND_RF VS_RF XTAL GND Pinning QFN24 5mm x 5mm 24 23 22 21 20 19 18 17 PA1 3 16 PA2 PB3/RESET 4 15 PA3/T0 PB2 5 14 PA4/USCK PA7 6 PA5/MISO 7 8 9 10 11 13 12 GND 2 PB1 ANT1 PB0 ANT2 PA0 CLK 1 PA_ENABLE VCC PA6/MOSI Figure 1-1. Atmel ATA5771/73/74 9214A-RKE-12/10 Table 1-1. Symbol 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 5 PB2 Function Port B is a 4-bit bi-directional I/O port with internal pull-up resistor/reset input 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 9 PA_ENABLE 10 ANT2 Emitter of antenna output stage 11 ANT1 Open collector antenna output Clock output signal for microcontroller. The clock output frequency is set by the crystal to fXTAL/4 Switches on power amplifier. Used for ASK modulation 12 GND 13 PA5/MISO 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 Ground Port A is a 4-bit bi-directional I/O port with internal pull-up resistor 21 VS_RF 22 GND_RF Transmitter ground 23 ENABLE Enable input 24 2 Pin Description Pin Transmitter supply voltage GND Ground GND Ground/backplane on exposed die pad Atmel ATA5771/73/74 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. VCC R Reset C GND The external pull-up resistor on the reset line must be selected so that high-voltage programming 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(R) 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 recommends using a 4.7k resistor as the lowest value. To guarantee the proper functionality of the debugWIRE, the pull-up resistor value must not go below 10k. The value of the pull-up resistor must therefore be between 10k and 50k. 3 9214A-RKE-12/10 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 0.01F 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 protecting 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. 4 Atmel ATA5771/73/74 9214A-RKE-12/10 Atmel Corporation 2325 Orchard Parkway San Jose, CA 95131 USA Tel: (+1)(408) 441-0311 Fax: (+1)(408) 487-2600 Atmel Asia Limited Unit 01-5 & 16, 19/F BEA Tower, Millennium City 5 418 Kwun Tong Road Kwun Tong, Kowloon HONG KONG Tel: (+852) 2245-6100 Fax: (+852) 2722-1369 Atmel Munich GmbH Business Campus Parkring 4 D-85748 Garching b. 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