PCVisionplus
Hardware Reference
4–1
Rev 02; February 8, 2002
CHAPTER 4
DAC and PLL PROGRAMMING
4.1 DAC REGISTERS
PCVisionplus contains two Dual-DAC (Digital to Analog Converter) devices. Each of the two devices contains two
13-bit DACs that control the ADC reference voltages, DC Restore (clamp) Voltage and the Sync Stripper Line Rate.
The DACs have a serial interface. The serial interfaces of these two devices are daisy-chained together, accessing
both devices using one serial port. The DACCS bit (INCON2 register) and the serial data bit in the DACPRG register
interface to and program the DACs.
4.1.1 DAC Interface
The 3-wire serial interface requires a Chip Select (CS, active low), Serial Clock (SCLK, active high), and Data In
(DIN). The DACCS bit (in INCON2 register) drives the CS pin. Programming DACCS to zero enables loading data.
Writing to the DACPRG register generates SCLK. The write pulse is connected to SCLK of the two DACs. Every
write to DACPRG causes a SCLK pulse. The DACSDATA bit in the DACPRG register is connected to the DAC
“Data In” bit (DIN) pin and clocked into the DAC by this SCLK pulse. The DAC “Data Out” pin is connected to the
DACSDATA bit in the DACPRG register. DACSDATA performs both write and read back functions.
Reading the DACSDATA bit is used in diagnostics to verify the serial daisy chain is connected. The internal DAC
values can not be read back through the DACSDATA bit.
Each data load command requires a 32-bit load, containing two 16-bit data values in an internal
shift register with the DACCS bit set low. When the DACCS bit is written high, the 32-bit command in the shift
register is transferred in and executed. Each time DACCS is brought low 32 bits must be loaded. After 32 register
writes to the DACPRG register with DACCS set low, the internal shift register is ready to execute the two loaded
commands by bringing the DACCS high.
NOTE
Use IFC library functions to program the reference DACs. These functions are written to handle
the timing and protocol of this serial interface.
All 32-bit data values use the following format:
0
15
A0
C1
C0
D12
D11 – D1
D0
16
32
A0
C1
C0
D12
D11 – D1
D0
PREF or NREF
PCLAMP or RATE
Load 32 bits at one time, starting with bit 31. Bit 31 is the first bit written to DACPRG; bit 0 is the last bit written. The
two 16-bit values access the two Dual-DACs in two distinct groups. The upper word always accesses either the
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