ZBasic System Library
283
ZBasic Microcontrollers
Type
Function returning Byte
Invocation
ShiftIn(dataPin, clkPin, bitCnt)
Parameter
Method
Type
Description
dataPin
ByVal
Byte
The pin used to input data.
clkPin
ByVal
Byte
The pin used to output a clocking signal.
bitCnt
ByVal
Byte
The number of bits to read in (1 to 8).
Discussion
This function can be used to input data from a synchronous serial device like a shift register. The pin
specified for input will be made an input but the pin specified for the clock signal must already be an
output and be at the desired initial logic level.
For each of the number of bits specified, then the clock line will be pulsed by changing its logic level
twice. The data line will be sampled approximately 2 CPU clock cycles after the leading edge of the clock
pulse. With a 14.7MHz CPU clock, this equates to about 135nS after the leading edge.
The returned value consists of the data bits read with the bit first read in the most significant bit position.
This is referred to as MSB first. If fewer than 8 bits are read, the low order bits will be zero.
Resource Usage
This subroutine uses the I/O Timer. If the I/O Timer is already in use, the function returns immediately
and the return value is zero. No other use of this resource should be attempted while the shifting is in
progress. Interrupts are disabled during the shifting process. However, RTC ticks are accumulated
during the shifting process so the RTC should not lose time.
Compatibility
For compatibility with I2C/TWI devices the clock rate is approximately 200kHz with
Register.TimerSpeed1 at its default value of 1. If the value of Register.TimerSpeed1 is changed,
the bit rate will be slower.
See Also
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