MCS(R) 51 FX/GX Microcontoller New Interrupt Priority Register INTEL REF NO: [2019][MIC1390]EFG PRODUCTS COVERED: 87C51FC, 8XC54, 87C51GB DATE/VERSION: 07/30/90; Ver 1.00 RELATED INFO: Data Book. KEYWORDS: MCS-51 FX/GX INTERRUPT ABSTRACT: This TechBit clarifies the name and function of the new interrupt priority register on the FX core. NEW INTERRUPT PRIORITY REGISTER NAME There has been some confusion about the name and function of the new interrupt priority register resident on the FX core. Figure 1 shows the correct naming convention for the register. IPH                 (0B7H) - PPCH PT2H PSH PT1H PX1H PT0H PX0H Figure 1 The new register becomes the MSB of the priority select bits and the existing IP register acts as the LSB. This scheme maintains compatibility with the rest of the MCS-51 family. Table 1 shows the bit values and the priority level associated with each combination. Priority Select Bits MSB LSB IPH.X IP.X (X is the bit position 0 - 6) 0 0 Interrupt Priority level 0 (lowest) 0 1 Interrupt Priority level 1 1 0 Interrupt Priority level 2 1 1 Interrupt Priority level 3 (highest) Table 1 To assure that all publications describing this feature are in complete agreement, an Automotive Handbook errata list is outlined below for the 87C51GB. Interrupt Priority Register as seen in the 1989 Automotive Handbook.   7 6 5 4 3 2 1 0 IP (0B8H) - PC PT2 PS PT1 PX1 PT0 PX0 IP1 (0B7H) - PC_1 PT2_1 PS_1 PT1_1 PX1_1 PT0_1 PX0_1 Interrupt Priority Register as seen in the 1990 Automotive Handbook.   7 6 5 4 3 2 1 0 IP (0B8H) - PC PT2 PS PT1 PX1 PT0 PX0 IPL (0B7H) - PCL PT2L PSL PT1L PX1L PT0L PX0L Correct Interrupt Priority Register for the 1990 Automotive Handbook.   7 6 5 4 3 2 1 0 IP (0B8H) - PC PT2 PS PT1 PX1 PT0 PX0 IPH (0B7H) - PPCH PT2H PSH PT1H PX1H PT0H PX0H The IP1 bit functions defined at the bottom of the Interrupt Priority Register Table should also be changed to match the IPH (0B7H) names from above. PC_1 should be PCCH PC2_1 should be PT2H PS_1 should be PSH PT1_1 should be PT1H PX1_1 should be PX1H PT0_1 should be PT0H PX0_1 should be PX0H

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