AN1139 STMicroelectronics, AN1139 Datasheet - Page 8

• Manufacturer Part Number: AN1139
• Description: L6254 - L6268 - L6269 12V DISK DRIVE POWER COMBO IC
• Manufacturer: STMicroelectronics

AN1139 APPLICATION NOTE
3.0 SERIAL INTERFACE CIRCUITS
3.1
The serial port interface is used to program the internal registers in addition to interfacing with the status and ID
registers. The serial port is enabled for data transfer when the Serial Data Enable pin (SDEN pin #16) is high
(=1). SDEN must be asserted high prior to any transmission and it should remain high until the completion of
the transfer. At the end of each transfer SDEN should be brought low (=0).
When SDEN is high, the data presented to the Serial Data pin (SDATA pin #17) will be latched on each rising
edge of the Serial Clock pin (SCLK pin #18). Rising edges of SCLK should only occur when the desired bit of
address or data is being presented on the serial data line. The data is latched into the internal register after the
16th rising edge of SCLK. Each transmission consists of a write control bit followed by a 7 bit address word and
8 bit data word. The address bits select the internal register to be written. The address and data fields are input
LSB first, MSB last, where LSB is defined as bit zero. When the write control bit is “1”, the contents of the read
register that is being addressed will be shifted out on the SDATA pin LSB first (positive SCLK edge). The SDATA
pin will switch from input (high Z) to driving output after the falling edge of the 8th SCLK pulse.
All write bits in the registers are reset whenever PORB is low except for Retract time Reg#9.3.6, Retract voltage
Reg#9.0.1 and 2/3 Phase Braking Reg#5.3 (this last one only for L6254) which must be set or reset by the ex-
ternal controller soon after power-up to insure that the correct value will be programmed in case of a power down
situation. Even though the Reg#8.2 bit is not used, it must be also reset (=0) by the external controller. The de-
vices have different input threshold values, the L6254 is 5V compatible, the L6268 and L6269 are 3.3V compat-
ible.
Figure 1. Serial Port Write Timing
Figure 2. Serial Port Read Timing
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SDEN
SCLK
SDATA
SERIAL PORT
SDEN
SCLK
SDATA
1
0
A0
1st Byte
A0
1st Byte
Serial Port Read Timing
Serial Port Write Timing
A1
A1
A6
A6
D0
D0
D1
D1
2nd Byte
2nd Byte
D2
D2
D7
D7