Windows Dialog Application - RDS generator
A self contained circuit that generates the generates RDS unique ID, PS, PTY and a simple Radio Text capability.
A PIC will synthesise a base band signal that will be applied to a balanced modulator
57 kHz clock is required for the modulator.
PIC based 16F84-20
A windows dialog based app will have 4 fields:
The field entries will be "sticky", stored in the windows registry.
A compile button will generate a PIC .asm file for subsequent assembly and "blowing". The compiled PIC code will be a simple sequencer. All the "tricky" CRC generation will be pre-calculated by the windows dialog based app.
| Pin 17 | (RA0) unused |
| Pin 18 | (RA1) unused |
| Pin 1 | (RA2) 57kHz |
| Pin 2 | (RA3) data (Manchester differential) |
| Pin 3 | (RA4) data rate clock (1.1875 kHz). |
Port B <0..6> is assigned as a 7 bit DTA. Bit 6 MSB, bit 0 LSB.
Pin 13 <7> Unused
57kHz * 256 = 14.592 MHz. This gives a straightforward division ratio. The instruction cycle time is 1/64 of 57 kHz, approximately 0.274 µs.
| 57 kHz Clock | Instructions | sequencer events | data period 0 | data period 1 | Time us |
| OUT 1 | |||||
| delay 8.77 µs | 32 | Nothing | Serialise and differentially encode | Nothing | 8.77 |
| OUT 0 | |||||
| delay 8.77 µs | 32 | Update DTA | Update DTA | Update DTA | 13.15 |
| OUT 1 | |||||
| delay 8.77 µs | 32 | Nothing | Manchester encode... acquire data Update data output to port |
Manchester encode... toggle data Update data output to port |
26.3 |
| OUT 0 | |||||
| delay 8.77 µs | 32 | Toggle data rate clock | Update clocks Trigger DTA if transition |
Update clocks Trigger DTA if transition |
35.07 |
| OUT 1 | |||||
| delay 8.77 µs | 32 | Update DTA go to data period 1 : 8 |
Update DTA go to data period 1 : 8 |
Update DTA go to data period 1 : 8 |
39.45 |
| OUT 0 0 0 | |||||
| delay 8.77 µs | 32 | Nothing | Index Message store | Index Message store | 52.6 |
16 of the above 96 instruction cycle sequences gives a data period of 0.842 ms.
RA4 data rate clock (1.1875 kHz). Rises 56 µs after data transition for a generous set up time.
The above sequence has 4 "useful" code execution periods. In place of a couple of instruction to output the port B levels and 14 or so NOP's- we can do something more useful.
I'll notionally label the four 16 instruction intervals as phase 1 to phase 4.
A series of events can now be pipelined. All events are, must, have got to be, no stupid arguments, precisely timed!
Every eighth data period acquire the next byte from the message store. Oh yes, indexing 260 bytes of data in a PIC16F84 is a piece of piss!
Every data period, serialise the message store byte. Here we also differentially encode the bit.
Every half data period, that's 8 of the 19kHz sequences above, we Manchester Encode the differentially encoded data. This means for a differentially encoded bit value of '1' for the data period we send out a '1' for the first half the period - then the inverse, a '0'. Conversely, for a differentially encoded bit value of '0' for the data period we send out a '0' for the first half the period - then the inverse, a '1'.
The Manchester encoded differential data is presented to RB3. This allows a generous set-up time before the clock pulse generated in phase 4.
The data rate and 2 * data rate clocks on Port B will be updated in this phase.
Present the data to the DTA via the cos (PI f td/4) filter on port B.
We are not synthesising the dsbsc 57kHz sub-carrier. The PIC only generates the modulation for the sub-carrier.
The application can be downloaded from here.
Set the fields and click generate. It will churn out something like:
|
; PI c123 ; PS >G8OSH< ; RT A message from nick@anotherurl.com. Enjoy the intermod! ; PTY 27 Oldies Music loop_1 ; Group 0B segment 0 ; c123 2bc 0b6c 10b c123 110 3e47 120 ; c1 23 af 02 db 10 bc 12 34 40 f9 1d 20 movlw 0xc1 ; 00 call sendita ; compensate for the extra goto movlw 0x23 ; 01 ... movlw 0x80 ; 10 call sendit movlw 0x80 ; 11 call sendit movlw 0xdc ; 12 call sendit goto loop_1 |
Each RDS group consists of 4 blocks of data and check word. These are concatenated in to a series of 13 bytes.
Save the above generated .asm file as message.asm. This is then "#included" into the PIC rds timing generator source.
I spent some time fathoming out the formulae used in the data-channel spectrum shaping.
PIC DTA output.
10 kHz per division. We're better than -60dB up to 50 kHz.
Change the PIC firmware and we have a 19kHz pilot tone generator.
This page was last updated on the 18th January 2003