9.3 PPS Support

All CryptoMemory® devices with user memory size 32Kb and larger support the Protocol and Parameter Selection (PPS) protocol, Section 7 of ISO 7816-3. This section only applies to these larger devices; CryptoMemory devices with memory sizes 16-Kbit or smaller do not support PPS.

At the end of an ATR sequence, subsequent to either a cold or a warm reset initiated by the reader, the device will be expecting either a ‘Class’ byte as part of a command header, or the initial character, PPSS, of a PPS request. If the device receives a byte = $FF, it will process subsequent incoming bytes as a PPS request. In all other cases, it will proceed with command processing. There are four bytes that comprise a PPS request or response:

Initial Character PPSS. Always equal to $FF.
Format Character PPS0. CryptoMemory supports two response values for PPS0, $00 and $01. See ISO 7816 for further definition of PPS0.
Parameter Character PPS1. Encodes Fn and Dn in the same manner as TA(1) in the ATR.
Checksum PCK.

The following table shows the PPS1 values supported by CryptoMemory devices with memory sizes 32Kb and above.

Table 9-1. PPS1 Values Supported by Microchip CryptoMemory®
Dl	Dl	f Max	4MHz	5MHz	5MHz
		Fl	0000b	0001b	1001b
		Fl	372	372	372
0001b	1	PPS1	01	11
		F/D	372	372
		baud rate	9600	9600
0010b	2	PPS1	02	12
		F/D	186	186
		baud rate	19200	19200
0011b	4	PPS1	03	13
		F/D	93	93
		baud rate	38400	38400
0100b	8	PPS1	04	14	94
		F/D	46.5	46.5	64
		baud rate	76800	76800	55800
0101b	16	PPS1	05	15	95
		F/D	23.25	23.25	32
		baud rate	153600	153600	111600
1000b	12	PPS1	08	18
		F/D	31	31
		baud rate	115200	115200

After the ATR, the reader will have the choice of proceeding with commands using default values of F=372 and D=1 (9600-baud at 3.5712 MHz), or negotiating values Fn and Dn through a PPS exchange. The following are four examples of PPS requests and responses:


Example:	We assume CryptoMemory® ATR contains the byte TA(1) = 15h, indicating that it is capable of using F=372 and D=16, leading to a baud rate of 153,600-baud at 3.5712 MHz. Assuming that this is the maximum speed supported by the device, the reader immediately attempts to set the F and D parameters that results in the specified baud rate. PPS_request = $FF $10 $15 $FA PPS_response = $FF $10 $15 $FA The newly negotiated values take effect immediately following this exchange, so the ETU, or duration of one bit on I/O, will now be 23 clock cycles instead of 372.
Example:	The reader insists on negotiating Fn and Dn equal to the default values, even though these would be used by default without a PPS exchange. There are two ways to do this: by sending PPS1_request = $11 or by not sending PPS1_request at all. PPS_request = $FF $10 $11 $FE PPS_response = $FF $10 $11 $FE or PPS_request = $FF $00 $FF PPS_response = $FF $00 $FF
Example:	The reader attempts to negotiate values that are not supported by the CryptoMemory device. In its response, the CryptoMemory proposes to continue with the existing F and D values by not sending PPS1_response. Even though new Fn and Dn values aren’t negotiated, this scenario is still considered a “successful” exchange according to ISO 7816. PPS_request = $FF $10 $45 $AA PPS_response = $FF $00 $FF
Example:	If the reader attempts to change the protocol to any protocol other than T = 0, such as T = 1, the CryptoMemory will indicate that it only supports T = 0. PPS_request = $FF $01 $FE PPS_response = $FF $00 $FF CryptoMemory will only operate at baud rates above the default 9600 baud through a successful PPS exchange. CryptoMemory cannot be set to higher baud rates through the use of a TA(2) byte in the ATR.