M494 datasheet(13/22 Pages) STMICROELECTRONICS

13 / 22 page

FUNCTIONAL DESCRIPTION (continued)

Skip Function

Program skip is implemented in the M494 by a

single memory bit associated with each program

word. The bit acts as a flag to the device to indicate

that the program word should be skipped and the

next program word read from the memory in as-

cending or descending order if the skip flag is set.

Programs are skipped only when accessed using

the memory sequence up/down commands. Direct

access to a program from the keyboard, RC or data

command sources will always override the skip

function. e.g. if skip is set on prog. 7 and prog. 7 is

commanded from RC then prog. 7 will be tuned

even if there is nc prog. stored in that memory

location.

In order to program the skip bit and to defeat its

function when required two commands are avail-

able : set skip flag and skip defeat. The skip defeat

switch is designed to be activated by a facia panel

on the TV set under which are infrequently used

controls.

On the set skip flag command the M494 stores the

current contents of the tuning, band and MS count-

ers and sets the skip bit. After the set skip flag

operation the contents of the tuning, band and MS

counters will not change and the device continues

to output these values. The operation is transpar-

ent to the user in terms of function but is indicated

on the display by the program number flashing at

5Hz for 1 second. In order to reset the skip bit for

any program word the desired program should be

selected with skip defeated. A station should then

be tuned, if required, and then the store command

issued. The store command automatically resets

the skip flag.

The skip defeat command enables the reading and

writing (plus resetting of skip flag) of memory words

whose skip flag is set. If skip is defeated the device

will only access the number of programs selected

by the option select i.e. If 16 programs only are

selected then skip defeat will NOT enable access

to all 20 programs.

Reset

There are two conditions under which the M494 is

reset : power on and start up (On command).

Power on reset is triggered whenever VDD falls

below about 3V.

The duration of this reset is 110ms after VDD has

been restored.

Power On Reset

(Powered down to Off or Standby states)

After the reset period of 110ms :

a) The program counter is set to program 1.

b) The outputs are disabled as defined in the Off

and Standby states. Se e Standby & Off

definitions.

c) The option selection, keyboard, momentary on

switch and, when in standby, the display, RC and

data inputs become active. For the ”activity

level” of the keyboard in Off and standby states.

See Standby & Off definitions.

d) An internal register is set to indicate that the

device has powered up from the powered down

state.

Start Up Reset

(Standby or Off states to On state)

A start up reset is instigated by the reception of the

commands given in the definition of terms or the

Standby section.

The following then occurs :

a) The internal register indicating that the device

has powered up from the powered down state

is read :

I) If the register is set than the memory word

addressed by the program counter is loaded

into the tuning counter and then the analog

values are read rom the memory.

II) If the register is reset then the previously

selected tuning and analog values are left

unchanged.

b) The AFC is defeated and the volume muted for

a period of approx 1.7 seconds or 0.6 seconds

longer than the other analog outputs.

c) The tuning and analogue outputs, except

volume, are enabled after approx. 1.1 seconds.

d) The volume output is enabled after 1.7 seconds

e) The standby output is pulled up internally to

logic HI

f) The internal register is reset.

If the device has either of the power up options

(power up in Off or standby states) selected then it

will perform a power up reset but all the outputs and

command sources will remain disabled, then on the

On command, a start up reset will be performed. If

the device is required to power up in the On state

using the momentary mechanical switch con-

nected to the h+i pin then it will perform an ordinary

power on reset followed immediately by a start up

reset. The outputs and command sources will be

enabled after the periods defined above.

M494

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