MICROPROCESSOR CONTROL
Chapter 16 described the mechanical control of a videorecorder deck by a microprocessor system. The control processor of a VCR has many functions beyond the deck, however, including those described already in this chapter. For tuning, mode control, system selection and signal-source selection it has much in common with the TV control system to be described now. It is based on the use of a microprocessor and an EEPROM (non-volatile) memory chip.
Figure 22.4 outlines a typical TV control system, largely applicable to VCRs too. The central processor is an 8-bit type deriving its clock pulses from crystal XL1. It has a mask-programmed ROM (16 kbyte) section containing its mode and operating instructions, customised for the make and model of receiver; and a small RAM (512 byte) for temporary storage of user instructions and other data.
At power-up the processor is reset, and then feeds stored factory set-up data along the I2C bus to all the peripheral ICs on it; these are ‘default’ settings which establish the initial values for brightness, volume, colour level etc. At the on command from the remote control the PSU section is switched on and the required broadcast channel data fetched from EEPROM memory, converted to an analogue level and fed to the tuner. The OSD section has a decoder/character generator which injects RGB signals into the video amplifiers. The text, symbols and graphics for these are generated within the control processor; we have seen in Chapter 8 how a complete teletext decoder can be incorporated in the control processor chip.
There are several variants of the control system fitted to different makes and models. The tuner may contain its own I2C decoder and D−A converter, and hook to the serial data bus like the other peripheral devices shown at bottom right of Fig. 22.4; there may be A−D converters for local keyboard scanning, a.g.c. processing etc., and separate control lines for panel-LED drive, audio/video muting, bandswitching etc., much depending on how much use is made of the I2C data bus elsewhere in the receiver.
EEPROM Memory
An essential part of the control system is the EEPROM (Electrically Erasable and Programmable Read-Only Memory) used to store all the operating data for the receiver and system in non-volatile form.
The data is factory preset but can be overriden by the technician (i.e. when resetting picture geometry after timebase repair) and by the user, for instance when the set is retuned. All communication is via the I2C bus, on which the memory chip has its own address at which serial data can flow in or out of it. Apart from the bus connections, then, the memory needs only supply and ground links, making for a simple package with as few as eight pins. Fig. 22.5 shows the internal architecture of an EEPROM chip with 8 kbit capacity. It is largely self-explanatory except perhaps for the high- voltage section at bottom right. This is an on-board voltage multiplier generating 20 V or 25 V from the 5 V Vcc supply, and is used when it is required to overwrite data in the memory core: the relatively high potential is used to ‘punch through’ to the floating gates of the memory cells, leaving charges which can typically be retained for many decades in the absence of any sustaining
potential, hence the term non-volatile. The A0, A1 and A2 lines at the bottom of the diagram are for chip-selection purposes in complex equipment, but are usually grounded in TV sets and video- recorders.