Low power, low pin count interface for an RFID transponder

1. A serial interface comprising:

a clock node;

a first bidirectional data port;

a second bidirectional data port; and

shift register circuitry coupled to the first bidirectional data port and the second bidirectional data port, the shift register circuitry including a first shift register to load data onto the first bidirectional data port, a second shift register to receive data from the first bidirectional data port, a third shift register to load data onto the second bidirectional data port, and a fourth shift register to receive data from the second bidirectional data port,

wherein the shift register circuitry is configured such that a leading edge and a falling edge of a clock signal associated with the clock node are used to shift data in each of the first shift register, second shift register, third shift register and fourth shift register.

2. A serial interface comprising:

a select node;

a clock node;

a first bidirectional data port;

a second bidirectional data port; and

shift register circuitry coupled to both data ports such that a leading edge and a falling edge of a clock signal associated with the clock node are used to shift or transfer data

wherein the shift register circuitry comprises:

first and second shift registers having an output coupled to the first data port;

third and fourth shift registers having an input coupled to the first data port;

fifth and sixth shift registers having an output coupled to the second data port;

seventh and eighth shift registers having an input coupled to the second data port;

a first inverter coupled between the clock node and the second shift register, fourth shift register, sixth shift register and eighth shift register; and

a second inverter coupled between an output of the first inverter and the first shift register, third shift register, fifth shift register and seventh shift register.

3. The serial interface of claim 2 wherein the first and second shift registers are clocked with alternative phases of the clock signal.

4. The serial interface of claim 2 wherein the third and fourth shift registers are clocked with alternative phases of the clock signal.

5. The serial interface of claim 2 wherein the fifth and sixth shift registers are clocked with alternative phases of the clock signal.

6. The serial interface of claim 2 wherein the seventh and eighth registers are clocked with alternative phases of the clock signal.

7. The serial interface of claim 1 further comprising a state machine coupled to a select node, the state machine being operationally coupled to the shift register circuitry.

8. The serial interface of claim 7 further comprising a data out register in bidirectional communication with the state machine.

9. The serial interface of claim 8 , wherein the data out register is coupled to the shift register circuitry.

10. The serial interface of claim 8 , wherein the shift register circuitry comprises a plurality of shift registers, the data out register being coupled to an input of one half of the plurality of shift registers.

11. The serial interface of claim 7 further comprising a data in register in bidirectional communication with the state machine.

12. The serial interface of claim 11 , wherein the data in register is coupled to the shift register circuitry.

13. The serial interface of claim 8 , wherein the shift register circuitry comprises a plurality of shift registers, the data in register being coupled to an output of one half of the plurality of shift registers.

14. A method of reading from a serial interface, the method comprising:

inverting a clock signal associated with a clock node to generate an inverted clock signal;

on a high transition of the clock signal using the inverted clock signal to drive a first bit of data associated with the first bidirectional data port from the first bidirectional data port to a first shift register and a second bit of data associated with the second bidirectional data port from the second bidirectional data port to a second shift register; and

inverting the inverted clock signal to generate a non-inverted clock signal and on a low transition of the clock signal using the non-inverted clock signal to drive a third bit of data associated with the first bidirectional data port from the first bidirectional data port to a third shift register and a fourth bit of data associated with the second bidirectional data port from the second bidirectional data port to a fourth shift register.

15. The method of claim 14 further comprising using the high and low transitions of the clock signal to drive an entire data word.

16. The method of claim 14 further comprising prior to reading from the serial interface selecting the serial interface using a chip select signal on the select node.

17. A serial interface comprising:

a clock node;

a first bidirectional data port;

a second bidirectional data port;

shift register circuitry coupled to the first bidirectional data port and the second bidirectional data port, the shift register circuitry including a first shift register to load data onto the first bidirectional data port, a second shift register to receive data from the first bidirectional data port, a third shift register to load data onto the second bidirectional data port, a fourth shift register to receive data from the second bidirectional data port, wherein the shift register circuitry is configured such that a leading edge and a falling edge of a clock signal associated with the clock node are used to transfer data in each of the first shift register, second shift register, third shift register and fourth shift register;

a data in register in bidirectional communication with the first and second bidirectional data ports through the shift register circuitry; and

a data out register in bidirectional communication with the first and second bidirectional data ports through the shift register circuitry,

wherein the first bidirectional data port and the second bidirectional data port are configured as a bidirectional pair for writing to or reading from the serial interface in parallel.