STV0680 datasheet(23/63 Pages) STMICROELECTRONICS

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Version 3.4

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STV0680B + VV6444/6410/6500

STV0680B hardware interfaces

4.5

UART module for RS232 interface

The UART module on STV0680B provides a 115200 baud full duplex interface to an external Host.

4.5.1

RS232 physical interface

STV0680B does not support direct connection to a serial link. Physical line driver circuitry is

required, e.g. using a standard RS232 transceiver chip, or by using a small number of low cost

discretes (as shown in

Chapter 6). The schematics shown assume a 3 core cable with a length of

no greater than 2m. Unscreened cable can be used, a suitable low cost connector is a 3.5mm audio

jack plugs/sockets.

4.5.2

UART module overview

Receiving data

The UART module in STV0680B receives serial data through the RXD pin. Data reception is

initiated by a 1-to-0 transition on RXD, and the received data is sampled every 8.66

µS.

If the RXD input is not 0 when the incoming data is first sampled, the UART module goes back to

look for another 1-to-0 transition. This is to provide rejection of false start bits. If the start bit proves

valid, reception of the rest of the frame will proceed.

Transmitting data

When a transmission is activated by STV0680B, the contents of an internal transmit shift register

are shifted onto the TXD pin, every 8.66

µS.

The UART operates at 12MHz, which is not an integer multiple of 115200, so the actual baud rate is

115232 +/- 0.3%, which is well within the requirements of a typical 16x oversampling UART, which

can tolerate a 3.75% error in baud rate.

4.6

Power management and battery type

STV0680B and the SDRAM require a 3.3V supply. Where a VV6410/6411 or VV6500 (3.3v) sensor

is used, a single voltage regulator can be used to regulate the supply for all three components from

the battery, or the sensor on-board VReg can be used. Where a VV6444 is used, the sensor

requires a 5V supply, hence a DC-DC converter is recommended to provide enough voltage when

the camera is powered from the USB bus. These are explained more fully in the reference design.

4.6.1

Power switching to USB

Included in the USB compliance requirements, is the ability of the camera to report to the PC when

connected to the USB bus, whether or not there is a fresh battery in the camera. This means that

power sourcing from USB is required, which also saves on battery life while the camera is used in

USB tethered video mode. In order to allow switching from battery to USB power, some additional

hardware is required (see also

Chapter 6):

The USBDETECT input to STV0680B is used to detect that a USB power supply is connected

If the USB is detected, additional hardware is used to switch off the battery

An inrush protection circuit can protect the USB bus from current draw in the case of a low

battery/battery not fitted. This may be necessary to comply with the USB specification.

For precise USB compliance requirements, please consult the USB specification version 1.1

4.6.2

Switched sensor supply

Dependant on the hardware implementation, the SENSPWR output from STV0680B can be used to

power off completely the sensor during Standby mode (in the case of VV6444) or put the sensor

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